render: modify background color for highlighted text when fg == bg

When rendering a selected/highlighted cell where the foreground and
background colors are the same, invert the background color to make
the selected text legible.

Closes #455

CHANGELOG.md

@@ -33,6 +33,10 @@
 * `generate-alt-random-writes.py --sixel`: width and height of emitted
   sixels has been adjusted.
 * _Concealed_ text (`\E[8m`) is now revealed when highlighted.
+* The background color of highlighted text is now adjusted, when the
+  foreground and background colors are the same, making the
+  highlighted text legible
+  (https://codeberg.org/dnkl/foot/issues/455).
 
 
 ### Deprecated

render.c

@@ -443,6 +443,11 @@ render_cell(struct terminal *term, pixman_image_t *pix,
         }
     }
 
+    if (unlikely(is_selected && _fg == _bg)) {
+        /* Invert bg when selected/highlighted text has same fg/bg */
+        _bg = ~_bg;
+    }
+
     if (cell->attrs.dim)
         _fg = color_dim(_fg);
     if (term->conf->bold_in_bright.enabled && cell->attrs.bold)