Remove unused and rarely used irrlicht color functions (#15442)

SColor.h contains many functions which are unused and/or perform linear
operations on non-linear 8 bit sRGB color values, such as the plus operator and
`SColor::getInterpolated()`, and there is no documentation about missing gamma
correction.
Some of these functions are not called or called only once:
* `getAverage(s16 color)`: Unused
* `SColor::getLightness()`: Unused
* `SColor::getAverage()`: Claims to determine a color's average intensity but
  calculates something significantly different since SColor represents
  non-linear sRGB values.
* `SColor::getInterpolated_quadratic()`: Claims to interpolate between colors
  but uses the sRGB color space, which is neither physically nor perceptually
  linear.
* `SColorf::getInterpolated_quadratic()`: Unused
* `SColorf::setColorComponentValue()`: Unused

Removing or inlining these functions can simplify the code and documenting
gamma-incorrect operations can reduce confusion about what the functions do.

This commit does the following:
* Remove the above-mentioned unused functions
* Inline `SColor::getAverage()` into
  `CIrrDeviceLinux::TextureToMonochromeCursor()`
* Rename `SColor::getLuminance()` into `SColor::getBrightness()` since it does
  not determine a color's luminance but calculates something which differs
  significantly from physical luminance since SColor represents non-linear sRGB
  values.
* Inline `SColor::getInterpolated_quadratic()` into `GameUI::update()`,
  where it is only used for the alpha value calculation for fading
* Document gamma-incorrect behaviour in docstrings

src/client/imagesource.cpp

@@ -566,7 +566,7 @@ static void apply_hue_saturation(video::IImage *dst, v2u32 dst_pos, v2u32 size,
 		for (u32 x = dst_pos.X; x < dst_pos.X + size.X; x++) {
 
 			if (colorize) {
-				f32 lum = dst->getPixel(x, y).getLuminance() / 255.0f;
+				f32 lum = dst->getPixel(x, y).getBrightness() / 255.0f;
 
 				if (norm_l < 0) {
 					lum *= norm_l + 1.0f;