svg external bitmap

This patch enables saving bitmaps in svg as external files, so as to reduce output size and rendering costs, and allow easier post-processing of bitmaps.

Currently only DCT images can be saved as jpg, other images are still embedded in svg.

It seems that `SplashBackgroundRenderer`'s constructor already fails if the format is not supported, so this functon has no chance to run. By 'independent' do you mean this function my be moved to another place in future?