sixel: add support for overlapping sixels

Writing a sixel on top of an already existing sixel currently has the
following limitations in foot:

* The parts of the first sixel that is covered by the new sixel are
  removed, completely. Even if the new sixel has transparent
  areas. I.e. writing a transparent sixel on top of another
  sixel *replaces* the first sixel with the new sixel, instead of
  layering them on top of each other.

* The second sixel erases the first sixel cell-wise. That is, a sixel
  whose size isn’t a multiple of the cell dimensions will leave
  unsightly holes in the first sixel.

This patch takes care of both issues.

The first one is actually the easiest one: all we need to do is
calculate the intersection, and blend the two images. To keep things
relatively simple, we use the pixman image from the *new* image, and
use the ‘OVER_REVERSE’ operation to blend the new image over the old
one.

That is, the old image is still split into four tiles (top, left,
right, bottom), just like before. But instead of throwing away the
fifth middle tile, we blend it with the new image. As an optimization,
this is only done if the new image has transparency (P1=1).

The second problem is solved by detecting when we’re erasing an area
from the second image that is larger than the new image. In this case,
we enlarge the new image, and copy the old image into the new one.

Finally, when we enlarge the new image, there may be areas in the new
image that is *not* covered by the old image. These areas are made
transparent.

The end result is:

* Each cell is covered by at *most* 1 sixel image. I.e. the total
  numbers of sixels are finite. This is important for the ‘mpv
  --vo=sixel’ use case - we don’t want to end up with thousands of
  sixels layered on top of each other.

* Writing an opaque sixel on top of another sixel has _almost_ zero
  performance impact. Especially if the two sixels have the same size,
  so that we don’t have to resize the new image. Again, important for
  the ‘mpv --vo=sixel’ use case.

Closes #562

CHANGELOG.md

@@ -103,6 +103,9 @@
   colors remain unchanged.
 * Tabs (`\t`) are now preserved when the window is resized, and when
   copying text (https://codeberg.org/dnkl/foot/issues/508).
+* Writing a sixel on top of another sixel no longer erases the first
+  sixel, but the two are instead blended
+  (https://codeberg.org/dnkl/foot/issues/562).
 
 
 ### Deprecated