OGL: Unbind the active VAO before attributeless rendering

In practice, this was s_ShowEFBCopyRegions_VAO on startup, with enabled vertex arrays but no data in the buffer (yet.)  For attributeless rendering, we don't want any bound / enabled arrays.

This change fixes a crash with older NVIDIA drivers that presumably crash when you try to reference VBOs without data or beyond their valid data.

Nevertheless, it should be correct.  AFAICT this is called only once per frame, so I don't expect any perf impact.

-[Unknown]