crash course: adds a feature unimaginatively named MFT_isDeferred, set in ProcessedPrePassMaterial::_determineFeatures, and checked for it's existsnce in features such as TerrainBaseMapFeatGLSL::getOutputTargets and ReflectCubeFeatGLSL::processPix to switch between output targets on an as-needed basis. since we're no longer checking for the presence of a texture specifically to make that determination, this will free us up to use deferred raw color later