Allow glBlitFramebuffer on desktop, avoid NV.

Only use NV if !GLES3.  There's no reason otherwise.
Support block transfers offset into a framebuffer.
Match the #ifdef for glBlitFramebufferNV.

It's defined in gl_state with these requirements.
Merge pull request #6140 from unknownbrackets/gpu-blocktransfer

Support offset blocktransfers, fix GLES3 check