Rewrite rendering pipeline

This was needed to fix various corner cases when doing blending of colors
in linear space. The new architecture has the same performance as the
old in the common case of opaque rendering with no UI layers or images.

In the case of only positive z-index images there is a performance
decrease as the OS Window is now rendered to a offscreen texture and
then blitted to screen. However, in the future when we move to Vulkan or
I can figure out how to get Wayland to accept buffers with colors in
linear space, this performance penalty can be removed. The performance
penalty was not significant on my system but this is highly GPU
dependent. Modern GPUs are supposedly optimised for rendering to
offscreen buffers, so we will see. The awrit project might be a good
test case.

Now either we have 1-shot rendering for the case of opaque with only ext
or all the various pieces are rendered in successive draw calls into an
offscreen buffer that is blitted to the output buffer after all drawing
is done.

Fixes #8869

docs/changelog.rst

@@ -115,6 +115,9 @@ Detailed list of changes
 - macOS: Add the default :kbd:`Cmd+L` mapping from Terminal.app to erase the
   last command and its output (:disc:`6040`)
 
+- Fix :opt:`background_opacity` being non-linear with light color themes
+  (:iss:`8869`)
+
 - Wayland: Fix incorrect window size calculation when transitioning from
   full screen to non-full screen with client side decorations (:iss:`8826`)