Proof of concept for shared OpenGL contexts

Call vsRenderer_OpenGL3::SetLoadingContext() to set a special shared
OpenGL context from another thread.  You may then call any desired
VectorStorm functions from that thread (typically, file/model loading
functions).  When you're done, call 'ClearLoadingContext()'.  It will
block until the driver finishes flushing any activities, at which point
any loaded data will be available for rendering from the main thread.

Mostly untested.  No testing whatsoever on whether this actually works
reliably cross-platform and on different hardware.

When you call