Added test which shows broken GL emulation when:
1. Binding to a VertexObjectBuffer and calling `glDrawArrays` on two
   different sections of it.
2. Using `glVertexPointer` with actual pointer to a vertex array
   and calling `glDrawArrays` on two different sections of it.
Merge branch 'vertex_array_object_bug' of github.com:michaeljbishop/emscripten into incoming
fix repeated calls to glDraw* where the count changes but not the client attributes; fixes part of #1589
handle glBufferData usages that are not in WebGL; fixes #1589