Provide a seperate Qt5 QPA plugin for eglfs, built against the raspberry pi's GL libs

The Qt5 package in the repos is a generic one built against mesa-libgl, so the standard eglfs plugin won't work on the raspberry pi. To work properly, it needs some more initialization code, which is provided in the Qt5 tarballs. This package extracts the eglfs plugin from the Qt5 source tree, renames it and builds it with the additional code.
With this, we can run QtQuick apps with hardware acceleration on the r-pi without recompiling the whole of Qt5.

Note: LD_PRELOAD'ing the r-pi's GL libs is still necessary (or symlinking them to /usr/lib/, but that might break other packages which depend on mesa-libgl).

I plan to also create a wrapper script which will do the LD_PRELOADing and select eglfs-rpb as the used platform plugin for a Qt5 app.