Consider how to handle multiple, different clear values for attachments (MRT)

beginPass() has one color clear value that applies to all color attachments, and one depth and stencil clear value.

Ideally there should be a way to specify a color clear value per color attachment. This maps well to most modern APIs, whereas for GL and GLES (3.0+) we'd likely begin using glClearBufferfv in the backend to implement htis.

(while we are at it, using QColor is not necessarily ideal always)