multiple render target support:

engine:
A) detection for independent bit depth render targets (meaning checking to see if a card can support writing to an RGBA8 and an RGBA16 simultaneously, or it it requires setting them to the same value)
B) fleshes out the naming conventions for the getOutputTargetVarName ShaderFeature method.

Note: does touch on https://github.com/GarageGames/Torque3D/issues/790. Will folks want that portion included in this PR, or shall we keep the opengl end to the opengl end?