sixel: fix sheared image when image crosses scrollback wrap-around

When a sixel image crosses the scrollback wrap-around, it is split up
into (at least) two pieces.

We use cursor->point.col for all pieces’ x-coordinate. This caused the
final image to appear sheared, since we do a carriage-return (after a
number of linefeeds) after each piece - this causes the cursor’s
position to be reset to the left margin.

The solution is simple; remember the cursor’s initial x-coordinate,
and use that to position all image pieces.

Closes #151.

CHANGELOG.md

@@ -63,6 +63,9 @@
 * Background opacity when in _reverse video_ mode.
 * Crash when writing a sixel image that extends outside the terminal's
   right margin (https://codeberg.org/dnkl/foot/issues/151).
+* Sixel image at non-zero column positions getting sheared at
+  seemingly random occasions
+  (https://codeberg.org/dnkl/foot/issues/151).
 
 
 ### Security

sixel.c

@@ -448,6 +448,16 @@ sixel_unhook(struct terminal *term)
     int pixel_rows_left = term->sixel.image.height;
     const int stride = term->sixel.image.width * sizeof(uint32_t);
 
+    /*
+     * Need to 'remember' current cursor column.
+     *
+     * If we split up the sixel (to avoid scrollback wrap-around), we
+     * will emit a carriage-return (after several linefeeds), which
+     * will reset the cursor column to 0. If we use _that_ column for
+     * the subsequent image parts, the image will look sheared.
+     */
+    const int start_col = term->grid->cursor.point.col;
+
     /* We do not allow sixels to cross the scrollback wrap-around, as
      * this makes intersection calculations much more complicated */
     while (pixel_rows_left > 0) {
@@ -478,7 +488,7 @@ sixel_unhook(struct terminal *term)
             .height = height,
             .rows = (height + term->cell_height - 1) / term->cell_height,
             .cols = (width + term->cell_width - 1) / term->cell_width,
-            .pos = (struct coord){cursor->col, cur_row},
+            .pos = (struct coord){start_col, cur_row},
         };
 
         sixel_overwrite_by_rectangle(