Build and package piglit with MinGW cross compilers.
This MR achieves does a few things:
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Build piglit with MinGW cross-compilers, using Waffle. Waffle allows to run more tests reliably (as it can force the piglit Windows to be any precise size the test demands, unlike GLUT/FreeGLUT)
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Package artifaces, but only on tags. See https://gitlab.freedesktop.org/jrfonseca/piglit/-/jobs/7991552 for example.
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Drop AppVeyor. We don't think there's any merit in keeping it around anymore.