Support the filterRes attribute on filters

The filterRes attribute sets the resolution for the intermediate images for filters thus helping with obtaining resolution-independent results.

It's not clear which intermediate images are supposed to have their resolution set to filterRes. All of them regardless of their actual size? Assume the filter effects region is filterRes size in pixels and then compute other intermediate image sizes from there? The way librsvg currently works is it uses the same size for all intermediate images (equal to the final output size), which offers a nice answer to this question, however I'm not sure this answer is correct because eventually filters will receive bitmaps exactly for their referenced inputs, located potentially off-screen.