sixel: do not reset palette after each image

A client can re-use the palette between images. Resetting the palette
breaks this.

Now we initialize the palette on demand, and resets it when the
palette size is changed (by the client).

CHANGELOG.md

@@ -59,6 +59,8 @@
   `\E[J3` was executed.
 * Assert (debug builds) when an `\e]4` OSC escape was not followed by
   a `;`.
+* Palette re-use in sixel images. Previously, the palette was reset
+  after each image.
 
 
 ### Security

sixel.c

@@ -12,10 +12,15 @@
 
 static size_t count;
 
+void
+sixel_fini(struct terminal *term)
+{
+    free(term->sixel.palette);
+}
+
 void
 sixel_init(struct terminal *term)
 {
-    assert(term->sixel.palette == NULL);
     assert(term->sixel.image.data == NULL);
     assert(term->sixel.palette_size <= SIXEL_MAX_COLORS);
 
@@ -26,11 +31,15 @@ sixel_init(struct terminal *term)
     term->sixel.param = 0;
     term->sixel.param_idx = 0;
     memset(term->sixel.params, 0, sizeof(term->sixel.params));
-    term->sixel.palette = calloc(term->sixel.palette_size, sizeof(term->sixel.palette[0]));
     term->sixel.image.data = malloc(1 * 6 * sizeof(term->sixel.image.data[0]));
     term->sixel.image.width = 1;
     term->sixel.image.height = 6;
 
+    if (term->sixel.palette == NULL) {
+        term->sixel.palette = calloc(
+            term->sixel.palette_size, sizeof(term->sixel.palette[0]));
+    }
+
     for (size_t i = 0; i < 1 * 6; i++)
         term->sixel.image.data[i] = term->colors.alpha / 256u << 24 | term->colors.bg;
 
@@ -139,9 +148,6 @@ sixel_delete_at_cursor(struct terminal *term)
 void
 sixel_unhook(struct terminal *term)
 {
-    free(term->sixel.palette);
-    term->sixel.palette = NULL;
-
     sixel_delete_at_cursor(term);
 
     struct sixel image = {
@@ -489,6 +495,10 @@ void
 sixel_colors_reset(struct terminal *term)
 {
     LOG_DBG("sixel palette size reset to %u", SIXEL_MAX_COLORS);
+
+    free(term->sixel.palette);
+    term->sixel.palette = NULL;
+
     term->sixel.palette_size = SIXEL_MAX_COLORS;
     sixel_colors_report_current(term);
 }
@@ -499,6 +509,9 @@ sixel_colors_set(struct terminal *term, unsigned count)
     unsigned new_palette_size = min(max(2, count), SIXEL_MAX_COLORS);
     LOG_DBG("sixel palette size set to %u", new_palette_size);
 
+    free(term->sixel.palette);
+    term->sixel.palette = NULL;
+
     term->sixel.palette_size = new_palette_size;
     sixel_colors_report_current(term);
 }

sixel.h

@@ -4,6 +4,8 @@
 
 #define SIXEL_MAX_COLORS 1024u
 
+void sixel_fini(struct terminal *term);
+
 void sixel_init(struct terminal *term);
 void sixel_put(struct terminal *term, uint8_t c);
 void sixel_unhook(struct terminal *term);

terminal.c

@@ -1168,6 +1168,7 @@ term_destroy(struct terminal *term)
     tll_foreach(term->alt.sixel_images, it)
         sixel_destroy(&it->item);
     tll_free(term->alt.sixel_images);
+    sixel_fini(term);
 
     free(term->foot_exe);
     free(term->cwd);