When clearing a multisampled buffer, the reactor blitter was only
clearing the first sample. This cl makes the reactor blitter
effectively clear all samples of a multisampled buffer.

Change-Id: I7ce1af401bf537fe85e1c8393a2c57144641ad8e
Reviewed-on: https://swiftshader-review.googlesource.com/18032Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>