bgo#703102 - Clip coordinates for rsvg_alpha_blt() in a more civilized fashion

The source offsets were not being validated correctly, so we could easily do a read or write
outside the bounds of the image surface.  We now use a generic function to clip rectangles
instead of doing it by hand.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=703102



Signed-off-by: Federico Mena Quintero <federico@gnome.org>