Work around sin(0) != 0 hw bug

It seems that the sin implementation doesn't return exact 0 for sin(0) and some dEQP cases converts sin(0) to bool and uses as a vertex attribute (something like dEQP-GLES3.functional.fragment_out.random.24).

We need to figure out how to work around this without incurring an (a == 0) ? 0 : sin(a) type workaround. Typically it's close enough to 0 that it doesn't matter but when it's converted to bool anything but exact 0 becomes true.