Traditionally, loading a malformed PNG file with
cairo_image_surface_create_from_png() resulted in
CAIRO_STATUS_NO_MEMORY. This was apparently changed in commit
c5ee3f11

 by adding CAIRO_STATUS_PNG_ERROR and using it for errors
returned from libpng. However, I don't see how this was supposed to
actually work.

There are a couple of error codes supported by
_cairo_surface_create_in_error(). Anything else is turned into
CAIRO_STATUS_NO_MEMORY, which is not a helpful status. For this reason,
CAIRO_STATUS_PNG_ERROR would be turned into CAIRO_STATUS_NO_MEMORY.

This commit adds support for CAIRO_STATUS_PNG_ERROR to
_cairo_surface_create_in_error() so that this error can actually be
returned to the caller.

Signed-off-by: Uli Schlachter <psychon@znc.in>