Wrap aliased RlColor in cpp.Struct

[jobf]

I was seeing error: cannot convert ‘Dynamic’ to ‘Color’ when passing a Color as a constructor argument.

A helpful bit of info from the haxe discord -

constructor arguments must be "dynamic compatible" on hxcpp which externs representing native structs won't be, try wrapping the struct in a cpp.Struct for the constructor argument

This explains why there are lots of this in the bindings -

typedef Foo = cpp.Struct<Bar>;

e.g. as seen in the commit, this allows Color to be passed as a constructor argument -

typedef Color = cpp.Struct<RlColor>;