mesa: bump MAX_VARYING to 32

mesa: bump MAX_VARYING to 32

We're starting to get apps utilizing more than 16 varyings and
most current hardware supports 32 anyway.

Tested with r600g.
swrast, softpipe and llvmpipe still advertise 16 varyings.

This fixes a WebGL crash after launching this demo:
https://developer.mozilla.org/en-US/demos/detail/falling-cubes

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54402

NOTE: This is a candidate for the stable branches.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>