Introduce image_init and image_free

Encapsulation, preparation for upcoming AVFrame switch.
Make image's data members private
Rewrite image implementation to use AVFrames internally
Use swscale for scaling, looks much nicer, is a bit slower, though.
Rename graphics.* to image.*
Use FFmpeg to write PNGs, drop libpng dependency
Choose a useful slitscan speed
Tool: Print less --help on errors