sws_utils: provide function to check whether a format pair is supported

Normally, input and output are orthogonal. But zimg may gain image
formats not supported by FFmpeg, which means the conversion will only
work if zimg is used at all. This on the other hand, depends on whether
the other format is also supported by zimg. (For example, a later commit
adds RGB30 output to zimg. libswscale does not support this format. But
if you have P010 as input, which the zimg wrapper does not support at
all, the conversion won't work.)

This makes such a function needed; so add it.

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