Patch for sprites not being marked as active when used in a SpriteCoordinateExpander

[MCRcortex]

Fix for #2601 class of issues, however it assumes that a new SpriteCoordinateExpander is created every frame. Marking it as active in the hot path every write might have performance implications, thus only a partial fix.

[jellysquid3]

I'm not really sure this is robust enough. At least, if you create a VertexBuffer from a BufferBuilder, and render that, it won't track the sprites. Can we perhaps put something into BufferBuilder.BuiltBuffer to track the sprites and have them be updated each time the buffer is rendered?