We don't need the OpenGL view to be transparent if the window itself
is not transparent. This has the potential to speed up the compositing
of the GL view onto the NSWindow.
This implements the basics for a GdkGLContext on macOS. Currently, rendering
only is fully working for the GskCairoRenderer case where we read back pixels
into a cairo surface for rendering. More work on synchronization is required for
the GL on GskGLRenderer case.
When we attempt to render a surface itself with GL, the context will ensure that
the new GdkMacosGLView is placed within the NSWindow. In other cases, we
use a dummy NSView and NSWindow for backing the NSOpenGLContext to
ensure that we can get accelerated drawing.
This gets GtkGLArea working when running with GSK_RENDERER=cairo.
