All resampling for GeglBuffers used to happen in RaGaBaA float, now
grayscale formats end up using YaA float instead, which will improve
performance of scaling/rotating in GIMP as well as other tools using
GeglBuffers resampling like the sculpt tool when in grayscale mode.

buffer: add members for interpolation bpp/components

buffer: make linear sampler component generic

buffer: generalize cubic sampler

buffer: generalize lohalo innerloop

For now, assuming that input is still a premultiplied format and treat last component
differently, should sniff whether the interpolation format has alpha and only do
different treatment in that case.

buffer: generalize nohalo for components

buffer: nohalo slight simplification

foo