[BZ#787092] Support "FillPaint" and "StrokePaint" as filter inputs

Submitted by Federico Mena Quintero

Description

See https://www.w3.org/TR/SVG/filters.html#FilterPrimitiveInAttribute for the spec about these attribute values.

The "in" and "in2" attributes in filter primitives need to support values of "FillPaint" and "StrokePaint", but rsvg_filter_get_result() doesn't handle them at all.

I think we need to create a temporary surface, the same size as the filter region, and fill it with the appropriate paint server. We can't assume a solid color because the paint servers could refer to gradients/patterns/etc.

(This is partly why we fail to render filters-composite-03-b.svg from the SVG test suite.)