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.)

[Rangi42]

Verbose logging (-vvvv) shows mismatched "Rel size" values between 32-bit MinGW and every other platform.

Correct log (most platforms): https://pastebin.com/EffXYRg9
Incorrect log (32-bit MinGW): https://pastebin.com/WFMW4S0X

	/*
	 * Computes the "relative size" of a proto-palette on this palette
	 */
	double relSizeOf(ProtoPalette const &protoPal) const {
		// NOTE: this function must not call `uniqueColors`, or one of its callers will break!
		double relSize = 0.;
		for (uint16_t color : protoPal) {
			auto n = std::count_if(RANGE(*this), [this, &color](ProtoPalAttrs const &attrs) {
				ProtoPalette const &pal = (*_protoPals)[attrs.protoPalIndex];
				return std::find(RANGE(pal), color) != pal.end();
			});
			// NOTE: The paper and the associated code disagree on this: the code has
			// this `1 +`, whereas the paper does not; its lack causes a division by 0
			// if the symbol is not found anywhere, so I'm assuming the paper is wrong.
			relSize += 1. / (1 + n);
		}
		return relSize;
	}

Note that I have previously rewritten this function to not depend on std::count_if or std::find, without affecting either logged output.