Make begin_frame() set a rendering colorstate and depth, and provide it
to the renderers via gdk_draw_context_get_depth() and
gdk_draw_context_get_color_state().

This allows the draw contexts to define their own values, so that ie the
Cairo and GL renderer can choose different settings for rendering (in
particular, GL can choose GL_SRGB and do the srgb conversion; while
Cairo relies on the renderer).