xwayland/ei: Dequeue events when all caps are available

Currently, we would start dequeuing events as soon as a device is resumed, regardless of its capabilities.

If the capabilities are not available, we would just fallback to the regular XTEST code path and not use input emulation.

As a result, it is very likely that we shall lose the first events until the compositor resumes first a device with the requested capabilities.

To avoid that issue, start emulating only once we have the capabilities we requested, if they match the seat capabilities.

Closes: #1732 (closed)