squash! Don't cast uvec4s as if those were floats

In TGSI, the STORE opcode takes the value to store in the second source.

Before 509f4633, we consulted sinfo->override_no_cast[1] for the non-image version, which is the appropriate source to check against for that case.

We also need to check avoid doing the value-cast when doing the image-version as well, so let's add that.

This fixes #21 (closed).