Blend: channels per pixel is no longer 4 in raw
This, essentially, reverts 78fc8af469a6bf783dda9140b5743b1341551dcb
Blend: _blend_colorspace_channels(): raw has 1 channel per pixel
This was probably missed (or it was not noticed) in 78fc8af469a6bf783dda9140b5743b1341551dcb

Fixes:
=================================================================
==19419==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7f31036b66e0 at pc 0x7f31cd73ce7a bp 0x7f3145815a80 sp 0x7f3145815a78
READ of size 4 at 0x7f31036b66e0 thread T57
    #0 0x7f31cd73ce79 in _blend_normal_unbounded /home/lebedevri/darktable/src/develop/blend.c:486
    #1 0x7f31cd744916 in dt_develop_blend_process._omp_fn.6 /home/lebedevri/darktable/src/develop/blend.c:2194
    #2 0x7f31c6ba5715 (/usr/lib/x86_64-linux-gnu/libgomp.so.1+0xd715)
    #3 0x7f31c9db80a3 in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x80a3)
    #4 0x7f31c68cdccc in clone (/lib/x86_64-linux-gnu/libc.so.6+0xe5ccc)

0x7f31036b66e0 is located 0 bytes to the right of 89644768-byte region [0x7f30fe138800,0x7f31036b66e0)
allocated by thread T10 here:
    #0 0x7f31cdbd4c9b in __interceptor_posix_memalign (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x54c9b)
    #1 0x7f31cd65c18d in dt_alloc_align /home/lebedevri/darktable/src/common/darktable.c:997
    #2 0x7f31cd731cd8 in dt_dev_pixelpipe_cache_get_weighted /home/lebedevri/darktable/src/develop/pixelpipe_cache.c:163
    #3 0x7f31cd7369dd in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:827
    #4 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #5 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #6 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #7 0x7f31cd732c54 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:814
    #8 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #9 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #10 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #11 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #12 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #13 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #14 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #15 0x7f31cd732c54 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:814
    #16 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #17 0x7f31cd732c54 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:814
    #18 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #19 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #20 0x7f31cd732c54 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:814
    #21 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #22 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #23 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #24 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #25 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #26 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #27 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #28 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689
    #29 0x7f31cd732452 in dt_dev_pixelpipe_process_rec /home/lebedevri/darktable/src/develop/pixelpipe_hb.c:689

Thread T57 created by T10 here:
    #0 0x7f31cdba3bba in pthread_create (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x23bba)
    #1 0x7f31c6ba5c56 (/usr/lib/x86_64-linux-gnu/libgomp.so.1+0xdc56)
    #2 0x7f31c6ba17d9 in GOMP_parallel (/usr/lib/x86_64-linux-gnu/libgomp.so.1+0x97d9)
    #3 0x3ed921823dcccccc

Thread T10 created by T0 here:
    #0 0x7f31cdba3bba in pthread_create (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x23bba)
    #1 0x7f31cd6fc149 in dt_control_jobs_init /home/lebedevri/darktable/src/control/jobs.c:512
    #2 0x7f31cd6f1945 in dt_control_init /home/lebedevri/darktable/src/control/control.c:317
    #3 0x7f31cd65fe3c in dt_init /home/lebedevri/darktable/src/common/darktable.c:745
    #4 0x400b7f in main /home/lebedevri/darktable/src/main.c:24
    #5 0x7f31c6809b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/lebedevri/darktable/src/develop/blend.c:486 _blend_normal_unbounded
Shadow bytes around the buggy address:
  0x0fe6a06cec80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0fe6a06cec90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0fe6a06ceca0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0fe6a06cecb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0fe6a06cecc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0fe6a06cecd0: 00 00 00 00 00 00 00 00 00 00 00 00[fa]fa fa fa
  0x0fe6a06cece0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0fe6a06cecf0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0fe6a06ced00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0fe6a06ced10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0fe6a06ced20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Contiguous container OOB:fc
  ASan internal:           fe
==19419==ABORTING