Use non-native opencl pow function

[jenshannoschwalm]

For integration test checks

[jenshannoschwalm]

@TurboGit we might try this to rule out native_powr() imprecisions

[TurboGit]

No change:

$ ./run.sh --op colorequal
Test 0158-coloreq-no-guided-filter
      Image mire1.cr2
      CPU & GPU version differ by 997354 pixels
      CPU vs. GPU report :
      ----------------------------------
      Max dE                   : 6.98332
      Avg dE                   : 0.21766
      Std dE                   : 0.37146
      ----------------------------------
      Pixels below avg + 0 std : 64.63 %
      Pixels below avg + 1 std : 87.49 %
      Pixels below avg + 3 std : 98.48 %
      Pixels below avg + 6 std : 99.74 %
      Pixels below avg + 9 std : 99.93 %
      ----------------------------------
      Pixels above tolerance   : 0.31 %
 
      Expected CPU vs. current CPU report :
      ----------------------------------
      Max dE                   : 1.37983
      Avg dE                   : 0.00848
      Std dE                   : 0.06640
      ----------------------------------
      Pixels below avg + 0 std : 98.05 %
      Pixels below avg + 1 std : 98.08 %
      Pixels below avg + 3 std : 98.19 %
      Pixels below avg + 6 std : 99.02 %
      Pixels below avg + 9 std : 99.69 %
      ----------------------------------
      Pixels above tolerance   : 0.00 %
 
  OK

Test 0159-coloreq-guided-filter
      Image mire1.cr2
      CPU & GPU version differ by 2.73934e+06 pixels
      CPU vs. GPU report :
      ----------------------------------
      Max dE                   : 12.86431
      Avg dE                   : 1.18455
      Std dE                   : 0.90806
      ----------------------------------
      Pixels below avg + 0 std : 67.59 %
      Pixels below avg + 1 std : 88.97 %
      Pixels below avg + 3 std : 97.73 %
      Pixels below avg + 6 std : 99.80 %
      Pixels below avg + 9 std : 99.98 %
      ----------------------------------
      Pixels above tolerance   : 9.09 %
 
      Expected CPU vs. current CPU report :
      ----------------------------------
      Max dE                   : 0.93404
      Avg dE                   : 0.00004
      Std dE                   : 0.00455
      ----------------------------------
      Pixels below avg + 0 std : 99.99 %
      Pixels below avg + 1 std : 99.99 %
      Pixels below avg + 3 std : 99.99 %
      Pixels below avg + 6 std : 99.99 %
      Pixels below avg + 9 std : 99.99 %
      ----------------------------------
      Pixels above tolerance   : 0.00 %
 
  OK

Total test 2

[jenshannoschwalm]

I modified the code for testing to do NO correction in the module, the problem is somewhere in the basic color space conversions it seems ...