Implement WebGL2's renderbufferStorageMultisample to allow anti-aliased FBOs
Revising to cover all new renderbuffer-related entry points and validation updates:
James already hooked up some if not all of this functionality in Bug 209098.
Taking this bug. Since this functionality isn't necessarily separable from that in Bug 126448, might end up duplicating this into that one and fixing both at the same time.
After work on this and Bug 126448 they're not easily separable. Since most of the work on this bug is done, duplicating this into the other bug.
*** This bug has been marked as a duplicate of bug 126448 ***