Replaced GApp::FilmSettings::enabled with FilmSettings::effectsEnabled() and FilmSettings::setEffectsEnabled()

[Michael Mara]

FilmSettings::enabled caused crashes when set to false in recent times due to design changes to GApp. 

To get most of the previous functionality of FilmSettings::enabled = false, you can now use FilmSettings::setEffectsEnabled() per-camera, either in your scene files, or programmatically.

We now make explicit in the documentation that we always allocate GApp::m_framebuffer and that is what we blit from if FilmSettings::effectsEnabled() is false.