Add GDK_GL env var and GdkGLFlags
This moves the GDK_ALWAYS_USE_GL env var to GDK_GL=always. It also changes GDK_DEBUG=nogl to GDK_GL=disable, as GDK_DEBUG is really only about debug loggin. It also adds some completely new flags: software-draw-gl: Always use software fallback for drawing gl content to a cairo_t. This disables the fastpaths that exist for drawing directly to a window and instead reads back the pixels into a cairo image surface. software-draw-surface: Always use software fallback for drawing cairo surfaces onto a gl-using window. This disables e.g. texture-from-pixmap on X11. software-draw: Enables both the above.
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