Enable GL-renderer in CI

The following discussion from !344 (merged) should be addressed:

@pq started a discussion: (+3 comments)
The reason this is WIP is that Mesa crashes when ran with llvmpipe:
```
LIBGL_ALWAYS_SOFTWARE=1 ./tests/test-subsurface-shot -f 2
```
causes a segfault in drisw.c put_image() calling into a NULL vfunc loader->putImage.

This happens with Mesa master branch of today, too.

Once a fix has been merged in Mesa master branch, adjust the Weston CI image to have a custom Mesa built from a git snapshot of Mesa and stop skipping the GL-renderer tests.

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