Strip alpha from inner skin parts

[Jerozgen]

Before:

After:

[IMB11]

I think we decided to keep support for alpha within skin textures as there are mods which users can install that allow for the alpha to work in-game (even if it doesnt work in vanilla) - will leave the decision to @AlexTMjugador though.

[Jerozgen]

I created this PR because my skin has transparency utilized by Ears Mod for wings texture, so the head was transparent in the launcher:

[AlexTMjugador]

For the Modrinth App, I think it makes the most sense to render skins as closely as possible to how the vanilla client does, which is a baseline most users will find reasonable. Since mods can change many aspects of skin rendering in countless ways, trying to account for those variations sounds both impractical and outside our scope.

When I first wrote this code, I chose not to implement transparency handling because the time required to fully understand Mojang's code and test the edge cases related to transparency didn't seem worth it, especially given that many third-party tools get it wrong anyway (so user expectations weren't clear to me, as @IMB11 noted, albeit for different reasons). However, the fact that someone submitted a PR to adjust the behavior to match the vanilla client is a strong signal that we should handle it that way 😄

[Jerozgen]

A skin texture buffer is now converted to RGBA. This simplifies other code and allows "Notch transparent hack" to add transparency even if the original skin image is RGB or Grayscale. Also I've added 2 more skins to the normalize_skin_texture_works test

[AlexTMjugador]

Nice work, thank you! ❤️

I think we can merge this after a few remaining nitpicks are addressed 🚀

[Jerozgen]

Done!