The previous implementation of the function had an incorrect comment
that implied XQueryTree was a great way to check if a Window is valid.
Any X11 function taking a Window is a good candidate, what we need to be
careful about is X11 exiting the program on any error.

Replace it with a call to  XGetWindowAttributes while ignoring X11
errors.

This fix was found while implementing similar functionality in Dawn.

Bug: dawn:269
Change-Id: I777e59cddbe94baf63286d11887b58c63ac2b66c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2001301Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>