Changing mode to a larger palette causes new palette to be calculated.

[bcampbell]

To reproduce:

1. Create a simple 16 colour pic with default palette (but only using a few colours)
2. Click on menu "edit->Change format..." and pick "32 colour palette"

Observed results:
A new 32-colour palette is calculated which contains only the colours used in the image. All unused colours are discarded and the image is remapped.

Expected results:
Old colours should be remain unchanged. New palette should just be extension of old one, likely with black in the new entries.