FCxtrans() returns int to conform with FC()
demosaic: speed up xtrans_markesteijn_interpolate()

Ideas, suggestions, and code with thanks to Ingo Weyrich:

- tune tile size
- don't add 6 to CFA offsets unless they're actually negative
- homogeneity buffers can reuse existing memory
- eliminate conditional when building homogeneity maps (gives 15%
  speed increase)
- pre-build 5x5 sums of homogeneity maps (another 15% speed increase)

Conflicts:
	src/iop/demosaic.c
demosaic: more Markesteijn clean-up/optimization

- as per Ingo Weyrich, set TS to 88 to try to keep data in a
  reasonable sized L2 cache
- cleanup buffer sizes/offsetes a bit more
- optimization from Ingo Weyrich, make yuv indexed by channel first
- use sizeof(uint8_t) in memset, even if it is 1
- for a few uint8_t-derived local variable, type them as uint8_t
tile min/max green calc, tile output

Use Ingo Weyrich's idea of a per-tile min/max green buffer, rather
than storing min/max in g1/g3 of intermediate image array. This allows
the intermediate array to be only 3 rather than 4 channels, reducing
memory consumption. This means no need to access the intermediate
image buffer during the rest of the tile processing, as the rgb[0]
buffer has the same data. Furthermore, write processed data directly
to the out[] buffer when have finished each tile, which saves a final
copy loop and gives an approx. 20% speedup.

allhex is now only useful in terms of tilesize as it only finds offsets
in rgb tile, not in the original image.
demosaic: fcol() offset note

row & col params can be -1 or -2; note this in comments as the
rationale for +6 then modulo 6

Conflicts:
	src/iop/demosaic.c

Conflicts:
	src/iop/demosaic.c
eliminate Markesteijn demosaic intermediary buffer

Additionally more speed-ups and better constants thanks to Ingo Weyrich.

Conflicts:
	src/iop/demosaic.c
simplify float X-Trans downscale/demosaic

get rid of code copied from Bayer version which isn't necessary as are
not aligning calculations to the sensor CFA
Merge pull request #631 from dtorop/markesteijn-improvements

demosaic: x-trans Markesteijn improvements