graphics/piglit: New port for the Mesa testsuite

Piglit is the Mesa testsuite. It is very helpful while working on the
graphics stack in both userland and the kernel.

Support for FreeBSD is incomplete at this time. We use a patch to
enable the same build flags as Linux. However, we still lack new code
to support FreeBSD, in particular in scripts under the framework
directory, such as a way to collect timestamped messages from the
kernel.

Approved by: kwm
Differential Revision: https://reviews.freebsd.org/D4058