These functions return the colour components as unsigned int and so
promote to this type in arithmetic expressions, unlike unsigned char
returned by the existing accessors without the "Get" prefix, which
promotes to (signed) int and results in gcc 12 -Warith-conversion
warnings when the result is then converted to unsigned, as it happened
in our own wxColour::GetRGB() and GetRGBA() functions and would probably
happen in a lot of code outside wx, which could also be updated to use
the new functions instead of inserting casts.
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