Scripting: Commands to read / write palettes

[Jengolus]

This PR adds scripting commands to manipulate color palettes and corresponding tests. They can be used as follows:

local palette = emu:readPalette(0)
print("Colors of palette 0:")
print("Index", "Red", "Green", "Blue")
for idx, color in ipairs(palette) do
    local values = util.unpackColor(color)
    print(idx, values.red, values.green, values.blue)
    -- Or equivalently:
    --print(idx, values[1], values[2], values[3])
end

local red = util.packColor(255, 0, 0)
-- Does nothing on GBA
emu:writePalette(0, palette[1], red, palette[3], palette[4])

Motivation: There isn't currently any way to directly modify palettes, especially for SGB games. In particular, I am working on a socket interface between the pixel art software Aseprite and mGBA to preview images directly in the games they should appear in, but I cannot change the colors on SGB.

Design choices:

• packColor takes red, green, and blue from 0 to 255, not 0 to 31
• unpackColor converts the channels as 255 * channel // 31, not 8 * channel
• writePalette accepts the colors as four integers and thus doesn't work for GBA. I tried to make it work using a table, but couldn't access the table's values from C. I can change that if given instructions.

[endrift]

First and foremost: There are several peculiar consistency issues with your code that imply you didn't review it yourself before submitting it. Is this code AI generated? Please review your own code for these issues before asking us to review it. Please also ensure you adhere to the existing coding and style guides used elsewhere in the code.

Second, when it comes to opinionated matters:

1. Do not scale the colors when reading/writing. Everyone has different opinions for how this should be handled, and it's a lossy operation, so we should not enforce it in the scripting API. The script itself can do whatever scaling it wants.
2. Writing palettes should only write a single entry in the palette, not the entire palette at once. It would solve your table issue (which itself is strange considering you construct a table in C already).

Please bring this PR to non-draft quality before asking for further review.

[endrift]

What is the logic behind returning 0x8000 here?

[Jengolus]

To my knowledge and testing, the highest bit is ignored, so I figured it would make a good error marker since readPalette cannot return a nil anymore.

[endrift]

This needs to be a LOAD_16.

[endrift]

This needs to be a STORE_16, but it also needs to be before the ->writePalette calls.

[Jengolus]

Done for both readPalette and writePalette.

[endrift]

Why did you remove mention of format and util.packColor?

[Jengolus]

I felt that mimicked the other docstrings more closely; added it back in now.