Original issue title: When using a background layer, switching to
Indexed Color Mode fills all layer bounding rectangles with
Color 0.
Conditions to reproduce the original issue:
- Opaque RGBA sprite, i.e. the bottom layer is 'Background'.
- There is a second layer with an ellipse (for example).
- There is a mask color #000000 alpha=0 is in the palette.
- The mask color index is greater and not equal than 0.
- Go to Sprite > Color Mode > Indexed.
Result: the transparent color of the second layer will change to
index color = 0 (usually black).
Also added test for RGBA->INDEXED conversion
5798e27993