Share Samplers for Occlusion/Roughness/Metalness/Opacity textures

With the PrincipledMaterial it is possible to use the same texture for:

OcclusionMaps

RoughnessMaps

MetalnessMaps

OpacityMaps

By setting the color channel which that property is sampled from.  While the same texture memory is used for each usage, a separate sampler is created for each property.  There is not an unlimited amount of Samplers available for materials at runtime, and we should not be so wasteful.

Instead we should find a way to use the same sampler when these material properties share the same texture source.