sixel: implement private mode 1070 - private color palette

When enabled (the default), sixels use private color registers. That
is, the color palette from the last sixel is *not* re-used.

When disabled, sixels share (i.e. re-use) the same color palette.

Closes #362

CHANGELOG.md

@@ -45,6 +45,8 @@
   `footclient` (https://codeberg.org/dnkl/foot/issues/337).
 * `-D,--working-directory=DIR` to both `foot` and `footclient`
   (https://codeberg.org/dnkl/foot/issues/347)
+* `DECSET 1070` - sixel private color palette
+  (https://codeberg.org/dnkl/foot/issues/362).
 
 
 ### Changed

csi.c

@@ -544,6 +544,10 @@ decset_decrst(struct terminal *term, unsigned param, bool enable)
         }
         break;
 
+    case 1070:
+        term->sixel.use_private_palette = enable;
+        break;
+
     case 2004:
         term->bracketed_paste = enable;
         break;
@@ -607,6 +611,7 @@ decrqm(const struct terminal *term, unsigned param, bool *enabled)
     case 47:   /* FALLTHROUGH */
     case 1047: /* FALLTHROUGH */
     case 1049: *enabled = term->grid == &term->alt; return true;
+    case 1079: *enabled = term->sixel.use_private_palette; return true;
     case 2004: *enabled = term->bracketed_paste; return true;
     case 27127: *enabled = term->modify_escape_key; return true;
     case 737769: *enabled = term_ime_is_enabled(term); return true;
@@ -646,6 +651,7 @@ xtsave(struct terminal *term, unsigned param)
     case 1047: term->xtsave.alt_screen = term->grid == &term->alt; break;
     case 1048: term_save_cursor(term); break;
     case 1049: term->xtsave.alt_screen = term->grid == &term->alt; break;
+    case 1070: term->xtsave.sixel_private_palette = term->sixel.use_private_palette; break;
     case 2004: term->xtsave.bracketed_paste = term->bracketed_paste; break;
     case 27127: term->xtsave.modify_escape_key = term->modify_escape_key; break;
     case 737769: term->xtsave.ime = term_ime_is_enabled(term); break;
@@ -684,6 +690,7 @@ xtrestore(struct terminal *term, unsigned param)
     case 1047: enable = term->xtsave.alt_screen; break;
     case 1048: enable = true; break;
     case 1049: enable = term->xtsave.alt_screen; break;
+    case 1070: enable = term->xtsave.sixel_private_palette; break;
     case 2004: enable = term->xtsave.bracketed_paste; break;
     case 27127: enable = term->xtsave.modify_escape_key; break;
     case 737769: enable = term->xtsave.ime; break;

sixel.c

@@ -18,7 +18,8 @@ static size_t count;
 void
 sixel_fini(struct terminal *term)
 {
-    free(term->sixel.palette);
+    free(term->sixel.private_palette);
+    free(term->sixel.shared_palette);
 }
 
 static uint32_t
@@ -46,17 +47,36 @@ sixel_init(struct terminal *term)
     term->sixel.image.height = 6;
     term->sixel.image.autosize = true;
 
-    if (term->sixel.palette == NULL) {
-        term->sixel.palette = xcalloc(
-            term->sixel.palette_size, sizeof(term->sixel.palette[0]));
+    /* TODO: default palette */
+
+    if (term->sixel.use_private_palette) {
+        if (term->sixel.private_palette == NULL) {
+            term->sixel.private_palette = xcalloc(
+                term->sixel.palette_size, sizeof(term->sixel.private_palette[0]));
+        } else {
+            /* Private palette - i.e. each sixel starts with a clean palette */
+            memset(
+                term->sixel.private_palette,
+                0,
+                term->sixel.palette_size * sizeof(term->sixel.private_palette[0]));
+        }
+
+        term->sixel.palette = term->sixel.private_palette;
+    } else {
+        if (term->sixel.shared_palette == NULL) {
+            term->sixel.shared_palette = xcalloc(
+                term->sixel.palette_size, sizeof(term->sixel.shared_palette[0]));
+        } else {
+            /* Shared palette - do *not* reset palette for new sixels */
+        }
+
+        term->sixel.palette = term->sixel.shared_palette;
     }
 
     for (size_t i = 0; i < 1 * 6; i++)
         term->sixel.image.data[i] = color_with_alpha(term, term->colors.bg);
 
     count = 0;
-
-    /* TODO: default palette */
 }
 
 void

terminal.c

@@ -1180,6 +1180,7 @@ term_init(const struct config *conf, struct fdm *fdm, struct reaper *reaper,
             .upper_fd = delay_upper_fd,
         },
         .sixel = {
+            .use_private_palette = true,
             .palette_size = SIXEL_MAX_COLORS,
             .max_width = SIXEL_MAX_WIDTH,
             .max_height = SIXEL_MAX_HEIGHT,

terminal.h

@@ -359,6 +359,8 @@ struct terminal {
         uint32_t alt_screen:1;
         uint32_t modify_escape_key:1;
         uint32_t ime:1;
+
+        uint32_t sixel_private_palette:1;
     } xtsave;
 
     char *window_title;
@@ -519,7 +521,9 @@ struct terminal {
         struct coord pos;    /* Current sixel coordinate */
         int color_idx;       /* Current palette index */
         int max_col;         /* Largest column index we've seen (aka the image width) */
-        uint32_t *palette;   /* Color palette */
+        uint32_t *private_palette;   /* Private palette, used when private mode 1070 is enabled */
+        uint32_t *shared_palette;    /* Shared palette, used when private mode 1070 is disabled */
+        uint32_t *palette;   /* Points to either private_palette or shared_palette */
 
         struct {
             uint32_t *data;  /* Raw image data, in ARGB */
@@ -528,6 +532,8 @@ struct terminal {
             bool autosize;
         } image;
 
+        bool use_private_palette:1;  /* Private mode 1070 */
+
         unsigned params[5];  /* Collected parameters, for RASTER, COLOR_SPEC */
         unsigned param;      /* Currently collecting parameter, for RASTER, COLOR_SPEC and REPEAT */
         unsigned param_idx;  /* Parameters seen */