qt: remove obsolete `qt_intf_t::voutWindowType`
... and only consider video-wallpaper
if
platform is windows.
We can not set an appropriate voutWindowType
during initialization, because it depends on
qt-compositor
which is evaluated later on.
For example, platform windows
might use
both dcomp
and hwnd
window types. In fact
none of the compositor integrations care about
qt_intf_t::voutWindowType
but rather set the
valid type themselves.
Fo example, compositor_dcomp
already uses
VLC_WINDOW_TYPE_DCOMP
and compositor_platform
already uses VLC_WINDOW_TYPE_HWND
, effectively
making qt_intf_t::voutWindowType
obsolete.