Refactor buffer clearing.

Bug 27460431

Change-Id: I90de3285c86c0d3187969270dcbd78a8b02feee9
Reviewed-on: https://swiftshader-review.googlesource.com/4978
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>