Draft: Fix "Drop Shadow" filter cutting off edges

There exists an issue with the Shadows and Glows -> Drop Shadow... filter in Inkscape stable and master versions where the resulting drop shadow gets cut off when the offset is too large compared to the source graphic:

Stable 1.0.2 & Master	Fixed

This issue arises because the Drop Shadow filter will apply an feFlood to the source graphic, compose it with the blurred source graphic, and then offset the result. Fixing this required restructuring the Drop Shadow filter to only offset the blurred source graphic but not the flood.

This PR also contains an overall refactoring of the source file for the Drop Shadow filter. I have confirmed that the refactored filter produces the correct results in Firefox and Chromium, with the example below:

Please take this merge request into account when considering my GSoC application. Thank you!