This is simply unnecessary and wastes time.
As part of this, simply remove the "all" directive. Only for a couple of tests
is it even potentially interesting to validate all pixels (e.g.
nointerpolation.shader_test), and for those "all" is replaced with an explicit
(0, 0, 640, 480) rect.
In all other cases we just probe (0, 0).
Values in DXIL have no signedness, so it is ambiguous whether 16-bit
constants should or should not be sign-extended when 16-bit execution
is not supported.
