shm: get_buffer(): make sure buffer->busy is set

When going through the cached buffers, we only set buffer->busy on
the *first* re-usable buffer we found.

In some cases, we will find more than one re-usable buffer. In this
case, we select the “youngest” one (i.e the one most recently used, in
the hopes that we can use damage tracking instead of re-rendering the
entire buffer).

If the “current” buffer is younger than the previously detected,
re-usable, buffer, then we unref:ed the previously selected buffer,
and replaced it with the current one.

But, we did not sanitize it. That is, we did not:

* set buffer->busy
* clear its dirty region
* clear its scroll damage

That buffer would eventually get rendered to, and committed to the
compositor. Later, the compositor would free it. And there, in our
buffer_release() callback, we’d assert that buffer->busy was
set. And fail.

Closes #844

CHANGELOG.md

@@ -60,6 +60,8 @@
   (https://codeberg.org/dnkl/foot/issues/842).
 * Key presses triggering keyboard layout switches also being emitted
   CSI codes in the Kitty keyboard protocol.
+* Assertion in `shm.c:buffer_release()`
+  (https://codeberg.org/dnkl/foot/issues/844).
 
 
 ### Security