Bypass scaling skip for mono 8 bit images

[jsouter]

With the mjpeg codec in use at Diamond, we have been seeing the following crash:

Assertion ((src_linesize) >= 0 ? (src_linesize) : (-(src_linesize))) >= bytewidth failed at src/libavutil/imgutils.c:350

for 8 bit mono images when entering the logic block that returns early from formatArray without performing sws_scale. This crash did not occur before the deprecated API calls were replaced. With some quick benchmarks on my workstation I couldn't see any appreciable change in cpu usage when bypassing this logic, @coretl and I discussed that the simplest solution may be to just scale all images to prevent this crash, though if there are other suggestions I can amend this - for the specific codec I have been testing with the crash can be avoided by setting the stride variable to 2 or 1.