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. The conversion between formats happens
at GTK level in GtkClipboard or, if GtkClipboard is not used, with
gtk_target_list_add_image_targets() to announce all supported image
formats, and with gtk_selection_data_set_pixbuf() to convert from
any GdkPixbuf formats to the format requested by the selection, and
with gtk_selection_data_get_pixbuf() to convert from the selection
format to GdkPixbuf, if supported.

GDK simply does not play any role in this. 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).