Blitbuffer optimization

Speed optimization mainly, by introduction of new N:M bpp blitting functions that can be implemented by choice. This should speed up things by some order of magnitude, especially to/from 4bpp buffers.

Note that this also drops RGB16, which we didn't and most likely won't ever use.

It introduces A8 format, which is what MuPDF returns.

mupdfimg is switched to use blitbuffer API instead of conversion.