cgifsave: remove regressions from 2853 (#2870)
cgifsave: remove regressions from 2853 (#2870)

* cgifsave: pick the best quantizer instead of the first good enough

* cgifsave: fix palettes comparison

* cgifsave: don't copy unchanged pixels to previous_frame
lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips