A cl recently introduced the XRB32F internal format, but while it
fixed some cases where this format is used as a rendertarget, it
seems to have caused regressions in sampling. A new case has been
added in the sampling code to handle this new format properly.

Change-Id: I7aa537a410d7b68c47771a72dec487e4b4c4e8be
Reviewed-on: https://swiftshader-review.googlesource.com/5147Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>