Fix ambient music looping

[WKFO]

I haven't got the slightest idea why no argument to loop isn't treated as such, but adding a default seems to fix #5049
It looks like otherwise the related code in SoundMusic.cpp gets a repeat = true (line 53 I think).

¯_(ツ)_/¯

[Web-eWorks]

@WKFO the relevant code is in src/lua/LuaMusic.cpp - it uses the pattern

bool repeat = true;
if (lua_isboolean(l, 2))
    repeat = lua_toboolean(l, 2) != 0;

Which should instead be bool repeat = LuaPull<bool>(l, 2, false);, that is, default to a false value if no argument is present. Changing all instances of the above resolves the root cause of the issue and also makes the code cleaner!

[WKFO]

@Web-eWorks saves me again. Thank you, and I pushed the change.