Before, cairo-util.h would combine pixman and cairo includes. X11 and
Wayland compositors uses this to load an image as a pixman_image_t but are
forced to include cairo headers. Clients use load_cairo_surface to
load images as cairo_surface_t's, but are forced to include pixman.h.
We move the load_image pixman prototype to its own header, so compositors
can get at the pixman prototype without including cairo.h and clients
can include the cairo based function without including pixman.h.
Add THEME_FRAME_MAXIMIZED flag so the theming system can know not to draw
shadows for maximized windows. This allows maximized surfaces' content to be
sized and placed in a more expectable fashion.
