This adds the following:
- ccs argument to GskRenderNode::draw
  This is the compositing color state to use when drawing.

- make implementations use the CCS argument
  FIXME: Some implementations are missing

- gsk_render_node_draw_with_color_state()
  Draws a node with any color state, by switching to its compositing
  color state, drawing in that color state and then converting to the
  desired color state.
  This does draw the result OVER the previous contents in the passed in
  color state, so this function should be called with the target being
  empty.

- gsk_render_node_draw_ccs()
  This needs to be passed a css and then draws with that ccs.
  The main use for this is chaining up in rendernode draw()
  implementations.

- split out shared Cairo functions into gdkcairoprivate.h
  gskrendernode.c and gskrendernodeimpl.c need the same functions.
  Plus, there's various code in GDK that wants to use it, so put it in
  gdk/ not in gsk/

gsk_render_node_draw() now calls gsk_render_node_draw_with_color_state()
with GDK_COLOR_STATE_SRGB.