Introduce dedicated OpenGL1 and OpenGL2 compositing types

The CompositingType enum turns into flags and two new values are
introduced: OpenGL1Compositing and OpenGL2Compositing.

Those new values are or-ed to OpenGLCompositing so that a simple check
for the flag OpenGLCompositing works in case of one of those two new
values. To make the generic check for OpenGL compositing easier a method
in EffectsHandler is introduced to just check for this.

The scenes now return either OpenGL1Compositing or OpenGL2Compositing
depending on which Scene implementation. None returns OpenGLCompositing.