This patch turns 'Color' from a class to a POD type by replacing
the constructors by the named create functions 'rgb', 'clamped_rgb',
and 'clamped_rgba'. It thereby enables the initialization of Color
values using { .r = ... } syntax and makes the type usable in const
expressions.
It also narrows the type for color components and alpha values to
uint8_t. So possible integer overflows of computed values are detected
by -Wconversion.
As 'Color::rgb(0, 0, 0)' is repeatedly used as a default value, the
patch adds the 'Color::black()' function.
Fixes#5238
bb06d879aa