meta/msaa-blit: consider weird sample count case unreachable

Suppresses a bunch of warning noise about sample_map possibly being used
uninitialized.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
vc4: Add a little bit more packet parsing to make dump reading easier.

Probably should have done this *before* staring at all those render lists
today.
vc4: Apply a Newton-Raphson step to improve RCP.

Fixes all the piglit floating-point *-op-div tests, among others.
vc4: Apply a Newton-Raphson step to improve RSQ

Fixes all the piglit built-in-functions/*sqrt tests, among others.