Fixed default numresolution for small JPEG2000 images

[radarhere]

Resolves #3161

A simple version of the problem in that issue is

from PIL import Image
image = Image.new("RGB", (1, 1))
image.save("test.j2k")

At the moment, it gives "OSError: encoder error -2 when writing image file".

Debugging, OpenJPEG is trying to say that the "Number of resolutions is too high in comparison to the size of tiles".

So this PR decreases the number of resolutions to work - unless the user explicitly set num_resolutions through a save argument.

If you're wondering where numresolution is set if Pillow's Python code isn't setting it, the default value for numresolution is 6 in OpenJPEG (or we might set the default to be 6 or 7).