Split filters into their own files
Closes #194 (closed).
This puts code related to each filter into its own C file at librsvg/filters/<filtername>.c
; I put the common filter-internal code into librsvg/filters/common.{c,h}
.
This leaves rsvg-filter.c
itself in a weird state where almost nothing goes in there; I left two public function implementations there.
There's also librsvg/filters/light_source.{c,h}
which shares some common code between specular_lighting
and diffuse_lighting
and also the light source node.
Please comment on what I need to move around or otherwise change.