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.

After all `HTMLRenderer::process` throws if `BackgroundRenderer::getBackgroundRenderer` returns null, may be we could let `getBackgroundRenderer` throw if a format is not supported?