VNG4 for Bayer sensors

This brings the Bayer-specific code from dcraw into linear and VNG
interpolate, so that it can work as VNG4 for Bayer images.

also misc. cleanups:
- lin_interpolate(): don't need to offset by roi_out x/y which is set to
  0,0 by process()
- vng_interpolate(): alloc a single memory range for both buffers
Merge pull request #627 from dtorop/vng4

VNG4 for Bayer sensors