gdk_cairo_draw_from_gl - fix drawing with alpha on window scale > 1

The scissoring needs to take into account the window scale.
gdk_gl_texture_from_surface - fix software fallback

We can't combine multiple draws into one for the software fallback,
because each quad has a different texture. And we generally don't
want to make a larger single texture because then we would have
to upload more data.
gdk: Fix scissoring with scaled windows

Make sure the window scale is always taken into acount when setting up
the scissor regions.