Prompt for downloading Minecraft if no jar is found

We get a lot of questions about red blocks (missing textures).

Instead of rendering with missing textures, chunky should notify the user and offer a button to download the latest stable jar and use that as a resourcepack.

We could even check the world version and tell if the current mc jar is too old

No files, tests, or entry points are identified in the issue. Start by locating the missing-texture fallback and the resource-pack or download UI; done means notifying the user, offering the latest stable Minecraft jar, and using it as a resourcepack, with version checking considered as an additional goal.

Written by the indexing model from the issue text.