imageop: tune up X-Trans bilinear downsample

don't offset up/left compared to VNG/Markesteijn
use floor of sample size for more sharpness
Merge pull request #639 from dtorop/xtrans-bilinear-tuneup

imageop: tune up X-Trans bilinear downsample
Histogram: replace dt_iop_roi_t with new dt_histogram_roi_t
Spots IOP: legacy_params(): adapt for latest mask changes
dt_masks_legacy_params(): refactor v1 -> v2 to a function, prepare for v3
Needed preparations for loading of uncropped, unscaled (B/W) raw images
rawprepare: new IOP, no-op (passthrough)
rawprepare: allow tiling

FIXME: verify this!
dt_imageio_open_rawspeed(): break everything! Stop scaling (B/W).
rawprepare: scaling for raw mosaic buffers
rawprepare: scaling for floating point buffers
Deflicker (histogram of: source raw data): fix (no scaling)
Disable "Create HDR": FIXME: this function needs adapting (scaling (B/W))
rawprepare: prepare for using blackLevelSeparate
rawprepare: raw mosaic: sepBlack
Rawprepare IOP: GUI
[FIXME] rawprepare: scaling for raw mosaic buffers [SSE]
rawprepare: scaling for floating point buffers buffers [SSE]

Conflicts:
	src/iop/rawprepare.c
Rawprepare iop: implement OpenCL kernel (1f)!
Rawprepare iop: implement OpenCL kernel (4f)!