dri3: For 1.2, use root window instead of pixmap drawable

dri3: For 1.2, use root window instead of pixmap drawable

get_supported_modifiers() and pixmap_from_buffers() requests both
expect a window as drawable, passing a pixmap will fail as the Xserver
will fail to match the given drawable to a window.

That leads to dri3_alloc_render_buffer() to return NULL and breaks
rendering when using GLX_DOUBLEBUFFER on pixmaps.

Query the root window of the pixmap on first init, and use the root
window instead of the pixmap drawable for get_supported_modifiers()
and pixmap_from_buffers().

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107117
Fixes: 069fdd5 ("egl/x11: Support DRI3 v1.1")
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>