This was already fast on Apple, but @Severin and @dfelinto noticed slowdowns in user prefs, which is text heavy.
The problem was immBeginAtMost not being smart about VBO write flushing. immBeginAtMost can use all of its allocated range or only a subrange. The previous code was forcing back-to-back draw calls and buffer writes to serialize. This commit lets OpenGL know that our VBO writes never overlap, so there's no need to wait.
Should be much faster now!
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