and gtk_snapshot_render_frame() to be direct replacements for the
old gtk_render_*() functions.
Use them to replace Cairo usage completely in gtk_window_snapshot().
This is in preparation for calc(), as calc(50% - 5px) is valid CSS
but has 2 units. Instead, add a function to query a value's dimension
(so we can differentiate lengths from numbers) and add a function to
query if the value contains percentages.
We skip sides with 0 border width in render_border, but when
we collect sides with the same style, we may pass the 0 width
down to render_frame_stroke anyway. So skip width 0 sides
there as well.
