`ktx create` should warn about images that exceed common GPU maximum texture dimensions

This stems from issue #373. Two things needs to be done:

1. Remove the artificial size limits in the JPEG decoder.
2. Due to GPU texture size limits, ktx should exit with an error if no --resize or --scale option is specified and the size is greater than 16,384 and it should issue a warning if the size is > 16,384 after a --resize or --scale. It should not automatically resize. Some users may want to resize to a different size having not been aware until warned that the image was so large.

Question: should `toktx` allow texture sizes > 32768. I doubt any GPU supports texture sizes larger than that.

Start by tracing the `ktx create` and `toktx` entry points, then inspect the JPEG decoder and the existing resize or scale handling. The work is complete when JPEG size limits are removed and oversized textures produce the specified error or warning without automatic resizing; the maximum accepted size still needs a decision.

Written by the indexing model from the issue text.