render: overlay: fix visual glitches when double buffering

When rendering the overlay for scrollback search, the logic assumed
buffer re-use. On some compositors this isn’t happening (on
e.g. KDE/plasma we’re forced to double buffer).

This resulted in matches not being highlighted correctly.

The problem is in how we calculated the region for which areas to
clear ("un-dim"). It uses the "previous frame’s see-through area"
minus the current frame’s see-through area.

However, when we’ve detected that the current buffer isn’t the same as
the last one, we set the last frame’s see-through region to "the
entire buffer". Thus, when calculating the diff, we end up with an
empty region, and nothing is highlighted.

Fix by simply using the current frame’s see-through region as-is when
we’ve detected we’re not re-using the last frame’s buffer.

CHANGELOG.md

@@ -74,6 +74,9 @@
 * Glitchy rendering when scrolling in the scrollback, on compositors
   that does not allow Wayland buffer re-use (e.g. KDE/plasma)
   ([#1173][1173])
+* Scrollback search matches not being highlighted correctly, on
+  compositors that does now allow Wayland buffer re-use
+  (e.g. KDE/plasma).
 
 [1173]: https://codeberg.org/dnkl/foot/issues/1173