Mode fallback / VrForward > Forward (#1805) * Add Mode fallback, used for both using Depth() as a fallback if shader doesn't have DepthNormal() and to move VrForward() to just Forward() * VrForward > Forward, reverse fallback so that old shaders with VrForward use that still * Remove duplicate Forward() and remove DepthNormal fallback * Put mode fallbacks on materialsystem init instead of gameinfo