fourcc: Add VLC_CODEC_SVG
codec/svg.c: Add svg decoder.
SVG decoder: various fixes

Various fixes:
- general cleanup
- fix wrong colors
- fix memleaks
- improve rsvg API usage
SVG decoder: Add options

Add the following options:
--svg-width
--svg-height The decoder tries to retain the source aspect ratio when scaling.
--svg-scale  The scaling value overrides the --svg-width and --svg-height for the image.
src/misc/image.c: recognize SVG images on mime-type and extension.
demux/image.c: Detect SVG Scalable Vector Graphics Images
Build SVG decoder
Add VLC_CODEC_BGRA fourcc.
SVG decoder: Use alpha channel.
codec/avcodec/swscale: Support VLC_CODEC_BGRA
vout_subpictures.c: Fix Dereference after null check (cid 1213781)

Fixes a bug introduced in commit 7d8714d90eaaf4f0e58685e62d14ac13d494f316