Fixed ridiculous ImageReader::computeFormat() implementation.

It was doing everything you shouldn't in a compute - having side effects, being non-threadsafe and being ridiculously slow. This knocks nearly 40% off the total runtime of an ImageReader->ImageTransform benchmark.

Also added a couple of todo items for the future.
Merge pull request #1141 from johnhaddon/imageReaderFix

Fixed ImageReader::computeFormat() implementation.