Only the 1st sample's buffer of multisampled buffers was getting
cleared instead of clearing all the samples' buffers, which led
to incorrect results. Added a loop to fix the behavior.

Change-Id: If22ebf2df61ae6afbc4c2975a9baee3c18a2492b
Reviewed-on: https://swiftshader-review.googlesource.com/4910Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>