FramebufferManager: Support stereoscopic EFB format changes.

Bonus stage, support obscure features with stereoscopy.

Format changes in the D3D backend were likely broken due to stereoscopic EFB copies. By adding stereoscopic support this bug is also fixed.
The fragcoord is by far not the same as written into gl_Position. I'd use another attribute instead of gl_FragCoord.z for the layer.