Blender Git Loki

UI: Perf: widgetbase: Replace imm usage by a batch cache.

Introduce a UI batch cache. For the moment it's only used by widgetbase so
leaving it interface_widgets.c. If it grows, it can have its own file.

Like all preset batches (batches used by UI context), vaos must be refreshed
each time a new window context is binded.

This still does 3 GWN_batch_draw in the worst cases but at least it does
not use the IMM api.

I will continue and batch the 3 calls together since we are really CPU
bound, so shader complexity does not really matters.

I cannot spot any difference on all the widgets I could test. I did not use
any unit tests so I cannot tell if there is really any defects.

This is not a complete rewrite but it adresses the top bottleneck found
after a profilling session.