svq1enc: Rename SVQ1Context to SVQ1EncContext

This allows making it visible without name clashes.
dsputil: Move SVQ1 encoding specific bits into svq1enc
dsputil: Move ff_alternate_*_scan tables to mpegvideo
dsputil: Move mspel_pixels_tab to the only place it is used
dsputil: Move APE-specific bits into apedsp
dsputil: Move Bink-specific add_pixels8 to binkdsp
dsputil: Split off quarterpel bits into their own context