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.

`SplashBackgroundRenderer()` already handles ENABLE_LIBPNG and ENABLE_LIBJPEG. The difference is that `SplashBackgroundRenderer()` throws if the format is not supported.