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
This improves texture pack compatibility. Masks are expected to be of the same
size as the base texture. This change upscales the smaller texture if needed.
The behaviour is now the same as a.png^b.png and a.png^[overlay:b.png (to mention a few).
