Multi-rect paint node ops
This is the last missing piece to transition St to paint nodes. This MR contains a couple of semi-related improvements, a preparation commit, and finally the new API. The new API is composed of:
clutter_paint_node_add_rectangles()
clutter_paint_node_add_texture_rectangles()
They map directly to cogl_framebuffer_draw_textured_rectangles()
.