pixelProgress[cluster].processedPrimitives could have been cleared
between the moment where it has been written and where it is read,
so I added a local variable to prevent this issue.

Change-Id: I495ed5c9e845c276890d6193b2fe4b8208f993d1
Reviewed-on: https://swiftshader-review.googlesource.com/5061Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>