Function get_coverage_source_svgfe 

pub fn get_coverage_source_svgfe(
    filters: &[(FilterGraphNode, FilterGraphOp)],
    surface_rect: LayoutRect,
) -> LayoutRect

Here we transform target rect to source rect for SVGFEGraph by walking the whole graph and propagating subregions based on the provided invalidation rect, and we want it to be a tight fit so we don’t waste time applying multiple filters to pixels that do not contribute to the invalidated rect.

The interesting parts of the handling of SVG filters are:

• scene_building.rs : wrap_prim_with_filters
• picture.rs : get_coverage_target_svgfe
• picture.rs : get_coverage_source_svgfe (you are here)
• render_task.rs : new_svg_filter_graph
• render_target.rs : add_svg_filter_node_instances