implement float->float downscale of xtrans raws

Lighttable view of xtrans images now works due to new function
dt_iop_clip_and_zoom_demosaic_third_size_xtrans_f(), parallel to Bayer
matrix's dt_iop_clip_and_zoom_demosaic_half_size_f().

also misc. cleanups:
- store xtrans matrix as uint8_t, rather than char
- rename dt_iop_clip_and_zoom_demosaic_onethird_size_xtrans to
  dt_iop_clip_and_zoom_demosaic_third_size_xtrans
- tighten up other xtrans downscale & demosaic code