Version in .obg file is hardcoded.

After downloading and installing OpenGFX 2 v0.8, my game still uses v0.7, because...

The baseset version number in the generated .obg file is hardcoded as 7.

This means that with multiple versions installed OpenTTD does not know which is actually the latest version, and will likely use a version with the lower number in its filename.

This number should be incremented for every published build.