qmlgl: fixes for building for android & mac with qmake
Since binaries of this plugin are not provided with gstreamer and cerbero also disables it by default, it is useful for the target users to be able to compile this with the build system that they know best: qmake.
This fixes building for Android and MacOSX with qmake and adds a README.md with instructions on how to do this exactly, since it requires setting certain environment variables for qmake.
I tested this for an Android build and a previous iteration was also tested for MacOSX. I expect it works the same also for Windows and likely for iOS, but I didn't feel like blindly adding the required ios:DEFINES += HAVE_QT_IOS as I have zero knowledge about this platform.