render: mark cell overflowed into as dirty

When tweak.allow-overflowing-double-width-glyphs=yes, then certain
glyphs are allowed to overflow into the neighbouring cell.

However, if the cell “owning” the double-width glyph is erased (_only_
that cell), then the cell overflowed into is not redrawn, causing
part of the double-width glyph to remain on screen.

To avoid checking for these glyphs when printing to the terminal (i.e
at parse time), simply mark both cells as dirty when we render the
overflowing glyph.

Yes, this means that the cells will always be re-rendered. We count on
them only making up a small portion of the screen.

CHANGELOG.md

@@ -32,6 +32,9 @@
   descriptor is closed.
 * Crash on compositors not implementing the _text input_ interface
   (https://codeberg.org/dnkl/foot/issues/259).
+* Erased, overflowing glyphs (when
+  `tweak.allow-overflowing-double-width-glyphs=yes` - the default) not
+  properly erasing the cell overflowed **into**.
 
 
 ### Security

render.c

@@ -481,7 +481,16 @@ render_cell(struct terminal *term, pixman_image_t *pix,
         col < term->cols - 1 &&
         (row->cells[col + 1].wc == 0 || row->cells[col + 1].wc == L' '))
     {
-        cell_cols = min(2, cols_left);
+        cell_cols = 2;
+
+        /*
+         * Ensure the cell we’re overflowing into gets re-rendered, to
+         * ensure it is erased if *this* cell is erased. Note that we
+         * do *not* mark the row as dirty - we don’t need to re-render
+         * the cell if nothing else on the row has changed.
+         */
+        row->cells[col].attrs.clean = 0;
+        row->cells[col + 1].attrs.clean = 0;
     }
 
     pixman_region32_t clip;