Fixed a few mistakes in the stencil clearing code to fix both
masking and scissoring. Also added a few variable in both the
depth clearing code and stencil clearing code to make them
more readable.

Bug 25971463

Change-Id: If33e2b0685fd35ef2edd31c56aea088cfbf2b430
Reviewed-on: https://swiftshader-review.googlesource.com/4335Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>