Searching in directory names can yield confusing results; e.g. if
the input is "jpeg2000/image1.jp2", it will infer the pattern
"jpeg%04d/image1.jp2", which is likely not what the user intended.

If the user really desires for the variable part to be in the
directory name, it can always use an explicit pattern.

Our prebuilt FFmpeg Windows binaries don't have PNG support enabled
(because that requires zlib), so that makes a PNG image a bad choice
for this test.

When FFmpeg doesn't support PNG, VideoCapture falls back to the
"image sequence" implementation, which doesn't work for single images.

at can't be a null pointer, so the condition was always false, and
a nonsensical pattern like "image.png%00d" was being inferred.