macos: use CGContext to draw cairo surfaces

macos: use CGContext to draw cairo surfaces

Instead of relying on cairo_t to perform drawing from our backing
image surface to the Core Graphics context, we can convert the
cairo_image_surface_t into a CGImageRef without having to copy
data if we are certain of the alignment of the image up front.

Without this, there are many situations, based on the size of the
window that could cause cairo to take a slow path and malloc/copy
the data to ensure that alignment.

The previous commit titled "macos: align image surface rowstride to
16-bytes" ensures that this invariant is true so that our drawing
code can assume we can reference the framebuffer from the
cairo_image_surface_t using a CGDataProvider.

Since GdkMacosCairoContext and GdkMacosCairoSubview are coordinating,
we can also setup the transformation/scale early when drawing the
cairo_image_surface_t instead of when copying it to Core Graphics.

Furthermore, the CGImageRef is created with an RGB colorspace so
that we are not performing colorspace conversion to the output
device. We don't get color matching between displays, but we don't
expect that anyway, particularly with the software renderer.