Split rsvg-filter.c into individual files
rsvg-filter.c is the largest C source file; it contains all the SVG filter effects.
Here we should put just the common infrastructure for filters - the base structs, the functions to store temporary results, and such.
Each implementation of a filter effect should be in a separate source file, probably librsvg/filters/filtername.c
. This will allow us to port one by one to Rust.