... and plumb the color state through the downloading machinery, where
no matter what path it takes it ends up in
gdk_memory_convert_color_state() or gdk_memory_convert().
The 2nd of those has been expanded to optionally do colorstate
conversion when the 2 colorstates are different.
Public headers should mainly include gdktypes.h, which already include
the symbol visibility and versioning macros; we can also modify
gdktypes.h to include the enumerations.
