Fix problem with images referenced within svg files. (bug#19373)

Fix problem with images referenced within svg files. (bug#19373)

Fixes: debbugs:19373
* src/image.c: Additional parameter 'filename' for
svg_load_image. Include "buffer.h".  Define library function
rsvg_handle_set_base_uri for WINDOWSNT.
(init_svg_functions): Initialize rsvg_handle_set_base_uri.
(fn_rsvg_handle_set_base_uri): Define fn_rsvg_handle_set_base_uri.
(svg_load): Pass a filename to svg_load_image: either name of
actual file or of current buffer's file.
(svg_load_image): New parameter 'filename', used for setting
base_uri, necessary for loading referenced images.