immBegin requires us to know how many vertices will be drawn. Most times this is fine, but sometimes it can be tricky. Do we make the effort to count everything in one pass, then draw it in a second?
immBeginAtMost makes this simple. Example: I'll draw at most 100 vertices. Supply only 6 verts and it draws only 6.
Any unused space is reclaimed and given to the next immBegin.
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