Introduce limited Xpm support (32 bit ZPixmap) for Cairo.

Introduce limited Xpm support (32 bit ZPixmap) for Cairo.

* configure.ac (HAVE_RSVG): Move after cairo.
(USE_CAIRO): Disable rsvg, don't disable Xpm.

* src/image.c (prepare_image_for_display): Don't load if USE_CAIRO.
(x_clear_image): If USE_CAIRO, also free possible img->ximg->obdata and
don't return early.
(ALLOC_XPM_COLORS): Don't define when USE_CAIRO.
(xpm_load): Convert simple Xpms (32 bit ZPixmap) to CAIRO_FORMAT_ARGB32
and create a surface.