swr: Enable MSAA in OpenSWR software renderer

This patch enables multisample antialiasing in the OpenSWR software renderer.

MSAA is a proof-of-concept/work-in-progress with bug fixes and performance
on the way.  We wanted to get the changes out now to allow several customers
to begin experimenting with MSAA in a software renderer.  So as not to
impact current customers, MSAA is turned off by default - previous
functionality and performance remain intact.  It is easily enabled via
environment variables, as described below.

It has only been tested with the glx-lib winsys.  The intention is to
enable other state-trackers, both Windows and Linux and more fully support
FBOs.

There are 2 environment variables that affect behavior:

* SWR_MSAA_FORCE_ENABLE - force MSAA on, for apps that are not designed
  for MSAA... Beware, results will vary.  This is mainly for testing.

* SWR_MSAA_MAX_SAMPLE_COUNT - sets maximum supported number of
  samples (1,2,4,8,16), or 0 to disable MSAA altogether.
  (The default is currently 0.)

Reviewed-by: George Kyriazis <[email protected]>

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