util: fix util_fill_rect to take util_color instead of u32 param

util: fix util_fill_rect to take util_color instead of u32 param

util_fill_rect could not handle formats with more than 32 bits,
since the fill color was a uint32_t value. Fix this by using
a util_color union instead, and also expand the union so it
works with formats which have up to 256 bits (the max of any
format currently defined).