In the image sequence capture, only search for the ordinal in the file name.

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.
Merge pull request #1786 from SpecLad:cap-images-file-pattern