qt: remove use of Qt's QOpenGLFunctions

We already have our own versions so use them instead so that we don't
have the potential for redefinition warnings between system GL headers
and Qt's internal headers.

Fixes gstreamer/gst-plugins-good#523
3 jobs for qt-build-gl in 6 minutes (queued for 3 minutes and 56 seconds)
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passed manifest #54062

00:00:25

 
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passed build fedora x86_64 #54063

00:02:41

 
  Full Builds
passed build android arm64 api28 #54064

00:02:53