This does nothing, since we have our own GL header.

They're not going to change until the next time the stream info is updated.
This would of course mainly be useful if we managed to do more than one or two
draws with the same stream info.

WINED3D_BUFFER_DISCARD means the (current) buffer contents are undefined for
subsequent operations. I.e., the map doesn't have to wait for any pending
operations to finish, and can just return a new buffer with undefined
contents. GL_MAP_UNSYNCHRONIZED_BIT means the driver doesn't wait for previous
operations to finish, and just maps a buffer that's potentially in use. Proper
synchronization is left to the application. Note that we set both
GL_MAP_INVALIDATE_BUFFER_BIT and GL_MAP_UNSYNCHRONIZED_BIT.
GL_MAP_INVALIDATE_BUFFER_BIT corresponds to WINED3D_BUFFER_DISCARD, and might
cause the driver to return a new buffer, but it's not required to make that
optimization.