Blender Git Loki

UI: (Performance) Avoid drawing buttons out of view

The UI was always drawing all buttons in a layout, no matter if they
were scrolled out of view (as in, outside of the visible part of the
region) or not. This means it's doing quite some work that can be
avoided.
UI drawing generally isn't a big bottleneck in Blender, so I don't
expect huge speedups from this. But while playing back animation, we do
redraw a fair bit of the UI, so in cases where there are many buttons
out of view, it may bring a little FPS boost. E.g. say in complex node
trees (the node editor is redrawn on animation playback in case there
are animated values that need updated UI feedback). This also mitigates
the issue in T92922 significantly.

Differential Revision: https://developer.blender.org/T92922

Reviewed by: Brecht Van Lommel