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.
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