Index color was changed in sprite areas that are painted with mask color after changing Transparent Color

[Gasparoken]

It's hard to me explain it with a few words, so I attach the action sequence to reproduce the bug:

1. Starting with a RGBA sprite with a mask color in the palette (mask color is index 2).
2. Convert the Sprite to INDEXED (index color areas added to the 2nd image).
3. Go to Sprite > Properties... > Transparent Color , pick color index 2.

Unexpected result: some sprite 'transparent areas' with color index = 0 changes to color index = 2 (lower part is transparent).
Expected result: the lower part of the sprite should be black.

This bug is a complement of issue #3207.

• Aseprite version: 1.2.34.1
• System: MacOS

[Gasparoken]

Implemented 4d075f7 to solve #3207, we get the following result:
RGBA test image:

Indexed image obtained (transparent color is now '1'):

Now, if we want to change the Transparent Color to 0 (through Sprite > Properties > Transparent Color), we can expect 2 different results:

1. Preserve the indices to keep the drawing intact. We'll get two mask colors (one defined as Sprite Transparent Color index=0, and other color which is transparent on index = 1). This option sounds good, but the artist may be confused by the existence of two mask colors.

2. Remap old transparent indices on the drawing to the new index 0. I like this, I think is better in the way nothing is hidden to the artist... but what happens if the artist wants to paint with Index = 1. I think the logical behavior is drawing with such index, so we'll get a drawing with 2 different mask colors.... finally I see option 1 is better.

Just for the records: current (1.2.35) conversion in similar circumstances:

The sprite Transparent Index was changed to 0, but mask color on the 2x2 image isn't changed. So, in the current behavior we also have 2 mask/transparent color on the sprite.

What do you think @dacap?