Switched rgb_t to a class, replacing macros with methods. Mappings are
as follows:

   MAKE_RGB(r,g,b)     == rgb_t(r,g,b)
   MAKE_ARGB(a,r,g,b)  == rgb_t(a,r,g,b)
   RGB_ALPHA(data)     == data.a()
   RGB_RED(data)       == data.r()
   RGB_GREEN(data)     == data.g()
   RGB_BLUE(data)      == data.b()
   RGB_BLACK           == rgb_t::black
   RGB_WHITE           == rgb_t::white

Implicit conversions to/from UINT32 are built in as well as simple
addition, subtraction, and scaling (with clamping).

As a result of being a class, some stricter typing was needed in
a few places but overall not too much.