use GL_TEXTURE_2D_ARRAY for most of our textures
Add geometry shader generator for stereo 3D.
Pass VS_OUTPUT structs between shaders.
VideoConfigDiag: Add Stereo 3D option.
PostProcessing: Add layered stereoscopy support.
VertexShaderManager: Compute stereoscopy projection matrices.
PixelShaderGen: Sample the correct texture layer.
TextureCache: Implement layered framebuffer support.

Stereoscopic EFB2Tex is now supported, EFB2RAM still requires layered texture conversion.
GeometryShaderGen: Allow stereoscopy to be disabled.

Will facilitate future use of this generator for other purposes.
TextureCache: Check the number of layers before reusing a texture.
TextureCache: Set proper vertex limit in geometry shader.

Without invocation 6 vertices are output instead of 3.