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VSE: Use early out for aplha over blending

When scaling down image, users expect to see background, which doesn't
currently happen in VSE. This is because strips use cross blend mode by
default, because alpha over is much slower. Reason is, because any area
of image can be transparent, and therefore it can't have early out
implemented in a way that cross blend mode can.

Flag images rendered by codecs that don't support transparency as fully
opaque and implement a form of early out for alpha over blend mode.

When rendering image stack, 2-input effects are ignored on the
"way down". Alpha over needs rendered overlay image to decide whether it
will use only overlay or background too. Therefore overlay can be
rendered safely before it is used. Image flags can be checked and it can
be freed if needed. Freeing doesn't cause any performance degradation,
because image is always stored in cache.

This feature does not improve blend mode performance. In summary, it
only allowes for having alpha over blend mode on background images
without suffering from lower performance.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D12914