Back off soft OpenGL 2.1 requirement

Background research discovered several old cards that support the
necessary VR mode extensions with OpenGL 2.0, so dropped requirement for
OpenGL 2.1.
Shader cleanup in OVR code paths

-Moved shaders into source file to prevent leakage.
-Changed behavior in bicubic to manually clamp to 0.0-1.0 range instead
of setting pixels outside that to black. This is sadly necessary to work
around a bug in AMD drivers where it doesn't respect GL_CLAMP_TO_EDGE.