fix support for non power-of-2 sized palette

[bcampbell]

There are a lot of places where power-of-2 size palettes are assumed.

Just to take one example:

1. load an image with a non-power-of-2 size palette, eg https://lospec.com/palette-list/twilight-5-32x.png
2. click "Edit -> Change Format..."
3. pick RGBA or RGB format, and click OK

Expected: image should now be RGBA or RGB

Actual: image is still indexed