Fix clearing all samples of multisample render targets.

Fix clearing all samples of multisample render targets.

Only libGLESv2 was clearing all the samples of a multisample buffer.
Since all known APIs always clear all the samples, this could be
handled in the Renderer.

Bug swiftshader:77

Change-Id: Ib9adc3c61d263420ed0a0ae4828a693bd360b076
Reviewed-on: https://swiftshader-review.googlesource.com/10788
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>