Histogram: replace dt_iop_roi_t with new dt_histogram_roi_t
Needed preparations for loading of uncropped, unscaled (B/W) raw images
dt_imageio_open_rawspeed(): break everything! Stop cropping & scaling (B/W)
rawprepare: new IOP, no-op (passthrough)
rawprepare: border cropping for raw mosaic buffers
Mipmap cache: prepare for border cropping for (downsampled) floating point buffers
rawprepare: border cropping for floating point buffers
rawprepare: scaling for raw mosaic buffers
rawprepare: scaling for floating point buffers
Deflicker (histogram of: source raw data): fix (crop + no scaling)
Disable "Create HDR": FIXME: this function needs adapting (cropping + scaling (B/W))
iop_dependencies.py: add rawprepare
Run iop_dependencies.py after merge!