vo_opengl: simplify screenshot code

Instead of reading back the image from textures, keep a reference to the
original image, and return that.

The main reason this was done this way was that originally, images
weren't refcounted, and would be deallocated or overwritten as soon as
the VO's draw call returned. But now there isn't really a good reason
for this anymore. One possibly _could_ argue that it was better because
other code could reuse the image sooner (e.g. for the cache), but on the
other hand, the VO runs already on a different thread, and filtering and
decoding each run on other threads too, so this argument probably
wouldn't hold up.

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