sixel: overwrite-by-rectangle expects ‘width’ to not exceed screen

This fixes a crash when the emitted sixel extends beyond the right
margin. The crash only happens when there are other sixel images
already emitted.

Fixes part of #151

CHANGELOG.md

@@ -61,6 +61,8 @@
 * Crash when `foot.ini` contains an invalid section name
   (https://codeberg.org/dnkl/foot/issues/159).
 * 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).
 
 
 ### Security

sixel.c

@@ -482,7 +482,8 @@ sixel_unhook(struct terminal *term)
         };
 
         sixel_overwrite_by_rectangle(
-            term, cursor->row, image.pos.col, image.rows, image.cols);
+            term, cursor->row, image.pos.col,
+            image.rows, min(image.cols, term->cols - image.pos.col));
 
         LOG_DBG("generating %dx%d pixman image at %d-%d",
                 image.width, image.height,