vout: update format after filters

Here is an updated (and rebased) version of this patchset, as discussed during the workshop this week.

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The vout is created first, based on the input format. Then filters (filter_t) could be added, possibly producing pictures in a different format.

      input ---> filters --->    MISMATCH    [I420] vout
            I420         RGBA

To avoid a mismatch between the output of the last filter and the expected format of the vout input, a converter was added when necessary to compensate:

      input ---> filters ---> converter ---> [I420] vout
            I420         RGBA           I420

But this was often a waste, and it caused problems for opaque formats.

Instead, request the vout to adapt itself to the actual format produced by the last filter. If it can, we can avoid an additional converter.

      input ---> filters --->    MISMATCH    [I420] vout
            I420         RGBA

      input ---> filters ------------------> [RGBA] vout
            I420         RGBA

If the vout does not support the new format (or does not accept to update its format), a converter is still added like before.