qt: x11: do not call `::{create,destroy}Fbo()` before making the context current and relax assertions

Rather than having an assertion regarding the context became current, we can simply early return in these functions. If the context could not be made current, it is likely that we are in some sort of transitional phase and resizeFbo() would be called later when the state is stable either directly or indirectly.

render(), on the other is periodically called anyway. It should be fine to simply return early instead of triggering assertion failure. When the context can be made current, it can do its job.

Request review @chub.