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

irr/src/CIrrDeviceLinux.cpp

@@ -1951,7 +1951,8 @@ Cursor CIrrDeviceLinux::TextureToMonochromeCursor(irr::video::ITexture *tex, con
 				XPutPixel(sourceImage, x, y, 0);
 			} else // color
 			{
-				if (pixelCol.getAverage() >= 127)
+				if ((pixelCol.getRed() + pixelCol.getGreen() +
+						pixelCol.getBlue()) / 3 >= 127)
 					XPutPixel(sourceImage, x, y, 1);
 				else
 					XPutPixel(sourceImage, x, y, 0);