Provide OpenGL over Egl

The Egl backend is decoupled from the OpenGL ES build option which makes
it possible to use it as a replacement for glx.

To make this possible a new build flag is added when egl is available at
compile time and any egl specific code is now ifdefed with this flag
instead of the gles flag. In addition at runtime a windowing system enum
value is passed to the various detect methods to have egl/glx specific
detection for e.g. function pointer resolving.

By default egl is used if compiled with OpenGL ES, otherwise glx is used.
But in the non-gles case the windowing system can be selected through the
new environment variable KWIN_OPENGL_INTERFACE. Setting this variable to
"egl" the EglOnXBackend is used.

REVIEW: 106632