[BZ#666666] use fallback resolution from the cairo context
Submitted by Christian Persch
Since the DPI is only used during rendering, rsvg should use the fallback resolution from the passed cairo_t's target, if set, instead of its own default DPI setting. That also makes it easier to use the same handle with different targets, e.g. rendering on screen, and printing, without having to re-set the DPI inbetween.
rsvg_handle_set_dpi[_x_y] and the rsvg_set_default_dpi[_x_y] should be deprecated, and the internal default be -1 meaning to consult the cairo target.
Also need to make sure to set the fallback resolution on the surfaces rsvg creates internally during rendering.