This is odd, but stencil masking was simply disabled, so
clearing the stencil buffer would only work if the mask
was 0xFF, or it would simply skip the clearing entirely.
I removed this condition to fix the issue. Also removed
some dead code and added an early exit if the mask is 0.

Change-Id: I359b10ed3382b75cb9d078470f237e68f1a6e7b9
Reviewed-on: https://swiftshader-review.googlesource.com/4303Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>