[radarhere]

• When comparing pixels from two P images, it is good to convert them to RGB first, since two P images might have the same indexes in their pixel data, but different values in their palettes. Yes, I checked that they didn't have any transparency, so they didn't need to be converted to RGBA.
• I found that when comparing I;16 images in Tests/test_file_jpeg2k.py, we can use assert_image_equal* rather than assert_image_similar*.

Once those two changes were made, convert_to_comparable() no longer did anything, so I've removed it.