Blender Git Loki

GWN: Vertex Buffer refactor.

We now alloc a vbo id on creation and let OpenGL manage its memory directly.
We use glMapBuffer to get this memory location.

This enables us to reuse and modify any vertex buffer directly without
destroying it with its associated Batches.

This commit does not really improve performance but will let us implement
more optimizations in the future.

We can also resize the buffer even if this can be slow if we need to keep
the existing data.

The addition of the usage hint makes dynamic buffers not a special case
anymore, simplifying things a bit.