VideoConfigDiag: Update descriptions.

Since we cannot accurately calculate the eye seperation or the focal length without headtracking these values are purely virtual.
TextureCache: Ensure that all render target textures have as many layers as the frame buffer.

Also fixes a case where the D3D code path did not initialize num_layers leading to undefined behaviour.
VideoCommon: Switch to Nvidia stereoscopy offset formula.
PostProcessing: Invalidate shader when anaglyph stereoscopy is toggled.
TextureCache: Add "Mono EFB Depth Copy" stereoscopy option.
OGL: Add warning message when stereoscopic 3D is enabled but unsupported.