backends: SDL: Avoid lockup in Wait.
backends: SDL: Avoid lockup in Wait.

Same story again. We cannot sleep in Wait while holding the mutex and we
do not have a file descriptor to poll on so we need resort to polling.

We will need a better solution here.

One posibility is to add a init flag to all backends and resort to
polling only if application needs thread safe backend.

Signed-off-by: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>

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