1. Use a graphene_vec2_t
2. Ensure it's always positive
3. Don't break with fallback
The scale value is nothing more than an indication of how many pixels to
assume per unit of a node.
Instead of tracking a single scale, track x and y scales separately.
Factor out gsk_vulkan_render_pass_new() into a private function that
receives both scales, and pass 'scale_factor' for both.
