Blender Git Loki

Fix performance issues with pose library sidebar and many poses

When the asset view in the sidebar of the pose library would contain
more than a few handful poses, interaction and animation playback
performance would be impacted considerably. This was because our icon
drawing scales image buffers using a rather slow method on the CPU.
This commit changes it so the asset icons are scaled using the GPU.

Note that this is a temporary change. I'd like all icon code to use
GPU-side scaling, see D13144. But such a change is too risky to do in
the release branch at this point, so this fix is specifically for the
3.0 release.