Use fractional scale for the GL renderer

Use fractional scale for the GL renderer

This commit combines changes in the Wayland backend,
the GL context frontend, and the GL renderer to switch
them all to use the fractional scale.

In the Wayland backend, we now use the fractional scale
to size the EGL window.

In the GL frontend code, we use the fractional scale to
scale the damage region and surface in begin/end_frame.

And in the GL renderer, we replace gdk_surface_get_scale_factor()
with gdk_surface_get_scale().