Simplify Wayland cursor theme handling

Now themes are loaded once per scale and stored centrally not per
window. They are not unloaded till application shutdown. Since there
is unlikely to be more than two or three scales this is not a big waste
of resources. Since cursor lifetime is tied to theme lifetime and cursors
are stored per window, destroying a theme requires destroying all
cursors win all windows referring to that theme, which is too much work.

Should hopefully fix #2810

docs/changelog.rst

@@ -64,6 +64,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - macOS: Make the window id of OS windows available in the ``WINDOWID``
   environment variable (:pull:`2877`)
 
+- Wayland: Fix a regression in 0.18.0 that could cause crashes related to mouse
+  cursors in some rare circumstances (:iss:`2810`)
+
 
 0.18.1 [2020-06-23]
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