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.