arb_tessellation_shader: Read tessellation levels in tessellation evaluation shader.

Test reading the tessellation levels written by the tessellation control
shader in the tessellation evaluation shader. Test this for all three
output primitive types. Also read tessellation levels that are not used by
the tessellation stage.
arb_tessellation_shader: Write per-patch output only in one thread.

Invoke tessellation control shaders with 16 threads. Write a per-patch
output varying only in one thread and test for it in the tessellation
evaluation shader.

The written value is dependent on gl_InvocationID to make sure that the
test fails if the GL fails to mask the write properly.
arb_tessellation_shader: Test that clipping uses correct clip distance.

Test that clipping uses the clip distance written by the tessellation
evaluation shader rather than that wirtten by vertex or tessellation
control shaders.
arb_tessellation_shader: Tests without tessellation control shader.

Test passing both user defined and built-in varyings from vertes shader to
tessellation evaluation shader with no (user defined) tessellation control
shader in between.
!UPSTREAM   demo tessellation shader