Needed preparations for loading of unscaled (B/W) raw images
dt_imageio_open_rawspeed(): better levels handling
Fixes DNGs from ML cr2hdr.

Thanks to <rwh> and <hal9k_> for making me notice it.
rawprepare: new IOP, no-op (passthrough)
rawprepare: allow tiling

FIXME: verify this!
dt_imageio_open_rawspeed(): break everything! Stop scaling (B/W).
dt_imageio_open_rawspeed_sraw(): 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)
dt_dev_pixelpipe_disable_{after,before}(): do not disable matching-one IOPs

pre:iopname now means: do not process any iops after iopname iop, and give us data after iopname iop
post:iopname now means: do not process any iops before iopname iop, start processing with first iop - iopname iop
[FIXME] Create HDR: adapt to scaling
Now we load raw data. Revisit creation code
rawprepare: prepare for using blackLevelSeparate
rawprepare: raw mosaic: sepBlack
Rawprepare IOP: GUI
rawprepare: scaling for raw mosaic buffers [SSE]
rawprepare: scaling for floating point buffers buffers [SSE]
Rawprepare iop: implement OpenCL kernel (1f)!
Rawprepare iop: implement OpenCL kernel (4f)!
iop_dependencies.py: add rawprepare
Run iop_dependencies.py after merge!
Needed preparations for loading of uncropped raw images