r_measureOverdraw is broken

[VReaperV]

It reads back a stencil buffer value and adds it to backEnd.pc.c_overDraw, but it's then never used. It might be better to do this in a different manner anyway, since this would mess with portals (or the other way around).

[VReaperV]

This is also not too useful with how it was implemented, since it will just give the amount of times pixels were modified, whether through depth pre-pass, or blended surfaces. A more useful approach would be to instead just add some constant value to the colour buffer using blend func add. For depth pre-pass we can just have a 2nd mode, which writes colour values during depth pre-pass as well (similar to how I've done with the subgroup profiler).