Support OpenGL 3.0 and 3.1 in the trigl3 test

[sorry, something went wrong with previous pull request]

The open-source Mesa radeon driver supports only OpenGL 3.1 on my GPU, with this change I can run the trigl3 test successfully.

For OpenGL <3.2 we should probably not request a core profile either (although it works on Mesa to do so).

OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD RV730
OpenGL core profile version string: 3.1 (Core Profile) Mesa 10.0.1
OpenGL core profile shading language version string: 1.40
OpenGL core profile context flags: (none)
OpenGL core profile extensions:
OpenGL version string: 3.0 Mesa 10.0.1
OpenGL shading language version string: 1.30
OpenGL context flags: (none)
OpenGL extensions: