pixels: truncate SDL_SetPaletteColors input array silently

[lyorig]

• I confirm that I am the author of this code and release it to the SDL project under the zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Makes SDL_SetPaletteColors return true even if the input array is too large, documenting the truncation behavior.

Description

This function currently has two failure conditions:

1. The palette is invalid.
2. The input array is too large.

Curiously, in the second condition, the colors that fit are still written, although false is returned. This seems somewhat confusing, and I'd suggest improving it in one of two ways:

1. Don't return false when this truncation occurs, and document that the length is adjusted automatically.
2. Keep the current behavior, and document that false doesn't necessarily mean that nothing happened.

This PR currently implements the former option (tests are passing), but I'd be happy to change it to the latter, since it is a somewhat opinionated change. I understand that the current behavior can help detect a size mismatch, since there is no public API for retrieving the size of an SDL_Palette. However, in either case, it's behavior that's worth documenting.

Existing Issue(s)

None.

[slouken]

Merged, thanks!