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.

I can catch exception from `SplashBackgroundRenderer()` and return null in getBackgroundRenderer(), but this looses the detailed error message.

I see that those exceptions are all caught  by main() and show the message there, so there is not much difference.