x11: be more verbose when glClientWaitSync behaves unexpectedtly

x11: be more verbose when glClientWaitSync behaves unexpectedtly

When given a 0 timeout, glClientWaitSync is only supposed to return one
of three possible values:

 - GL_ALREADY_SIGNALED - fence fired
 - GL_WAIT_FAILED - there was an error
 - GL_TIMEOUT_EXPIRED - fence hasn't fired yet

In addition, it can also return GL_CONDITION_SATISFIED if a non-zero
timeout is passed, and the fence fires while waiting on the timeout.

Since commit 972134abe48a4c9c7b6ad41b0723f30f4e7ae16b we now call
glClientWaitSync (with a 0 timeout), but one user is reporting it's
returning some value that's not one of the above four.

This commit changes the g_assert to a g_error so we can see what
value is getting returned.

May help with https://gitlab.gnome.org/GNOME/gtk/-/issues/2858