Optimize geoshader Sprite buffer writes into VertexDomain

[cspotcode]

Extracted from #905

Adds an optimized code path for writing values into a VertexList. Sprite and other pyglet internals can use this code path.

This code path can only be used by things which exclusively control their VertexList, because they must know every time the VertexList is migrated. This is true for Sprite, since _vertex_list is private, and the only way to migrate a Sprite's VertexList is by calling Sprite methods or properties.

Makes code changes for experimental geoshader Sprite to use this optimized code path.

Command	Mean [s]	Min [s]	Max [s]	Relative
Before: 448a3ee4 Fix shader (pre-existing bug in geoshader Sprite, unrelated to my changes)	317.981 ± 2.675	315.556	320.851	1.70 ± 0.02
After: 31c607fc Optimize buffer writes from geoshader Sprite into VertexDomain	186.733 ± 0.799	185.929	187.527	1.00