GDK W32: Be honest about supported clipboard formats

GDK W32: Be honest about supported clipboard formats

Do not lie to W32 about the formats that we provide or accept.

Originally the logic behind such lies was that GdkPixbuf allows us to
convert any supported image to BMP or PNG, and therefore we should
announce that we always provide/accept BMP and PNG along with other
formats.

But that's not how it works. GDK has built-in serializers and
deserializers for all pixbuf formats (where it just invokes GdkPixbuf
API) and will use them automatically to read or write GdkTexture
objects (internally wrapping GdkPixbuf objects where necessary). The
encoding and decoding of images is handled
by GdkContent(De)Serializers, backend has nothing to do with it.

Therefore W32 GDK backend should only offer formats that it can
actually do conversion for by itself (such as image/bmp <-> CF_DIB,
or text/uri-list <-> CFSTR_SHELLIDLIST).