sws_utils: work around libswscale corrupting memory yet again

If the alignment is less than 16, certain libswscale code paths will
silently corrupt memory outside of the target buffer. This actually
affected the libmpv software rendering API (that was fun to debug).
Rather than passing this problem to the next API user, try to avoid it
within libmpv.

It's unclear which alignment libswscale requires for safe operation. I'm
picking 32 (one more than the observed safe value in the case I
experienced), because libavfilter mostly uses this value.

The way to work this around is slow: just make a full copy of the entire
input or output image. Possibly this could be optimized by using the
slice API, but that would be more effort, and would likely expose
further libswscale bugs. Hope that this is a rarely needed path.

The next commit will update the alignment requirement documentation
bits.

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