glmark2 is a popular OpenGL (2.1 / ES 2.0) benchmark on Linux. Let's add
its shaders. The shaders are licensed under the GPLv3
(https://github.com/glmark2/glmark2/blob/master/COPYING)

v2: Update COPYING to include GPL notice

Note: the shaders themselves don't have copyright notice, so the authors
are from grepping the main source tree:

	$ grep -hr Copyright *.cpp *.h | sort -h | uniq

I'm assuming nobody wrote shaders without touching the main tree.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>