vo_vdpau: don't try to create surfaces of size 0

At least on kwin, we decide to proceed without waiting for the window
being mapped (due to the frame exts hack, see commit 8c002b79). But that
leaves us with a window size of 0x0, which causes VdpOutputSurfaceCreate
to fail. This prints some warnings, although vo_vdpau recovers later and
this has no other bad consequences.

Do the following things to deal with this:
- set the "known" window size to the suggested window size before the
  window is even created
- allow calling XGetGeometry on the window even if the window is not
  mapped yet (this should work just fine)
- make the output surface minimum size 1x1

Strictly speaking, only one of these would be required to make the
warning disappear, but they're all valid changes and increase robustness
and correctness. At no point we use a window size of 0x0 as magic value
for "unset" or unknown size, so keeping it unset has no purpose anyway.

CC: @mpv-player/stable