Changed to lerp-based blending, with an implicit base layer.
Renamed some variables to be more clear (blendWeight instead of blendMask, since the weights could come from vertex colors instead of a texture).
Updated DefaultBlendMask_layers.png to better suit the new layering model. It has a black background and overlapping R, G, and B areas.
Updated some of the test materials UV transforms to better fit the new DefaultBlendMask_layers image.
Added a new test object, which is a plane that has painted vertices.
Note that I updated test criteria in AtomSampleViewer to account for these changes as well.
Chris Santora
89bb0edb30