Add color_curve RGBGFX test

[Rangi42]

Follow-up to #1553. Adds a test for -C/--color-curve support, which is breaking somehow on 32-bit MinGW.

color_curve.out.pal result.pal differ: char 1, line 1
00:0000 
< 00:0000: ec1f 0516 0748 0000 ff03 e03b 0c7f 0516  .....H.....;....
< 00:0010: ff03 ec1f e075 0000 ff03 ec1f 0748 0000  .....u.......H..
< 00:0020: ff03 1f57 0c7f e075 ff03 3a77 1f57 0c7f  ...W...u..:w.W..
< 00:0030: 9f02 e075 2e09 0000 9f02 2e09 0748 0000  ...u.........H..
---
00:0000 
> 00:0000: ff03 ec1f e075 0000 ff03 e03b 0c7f 0516  .....u.....;....
> 00:0010: ff03 3a77 1f57 0c7f ff03 1f57 0c7f e075  ..:w.W.....W...u
> 00:0020: 9f02 2e09 0748 0000 9f02 e075 2e09 0000  .....H.....u....
> 00:0030: ff03 ec1f 0748 0000 ec1f 0516 0748 0000  .....H.......H..

(The palette order changes from 0 1 2 3 4 5 6 7 to 2 1 5 4 7 6 3 0.)