Convert the global offset from seconds to beats before using it in TimingData calculations

[sukibaby]

I've listed @phantom10111 as a co-author on this commit for their work on PR #300. Thank you!

The theory here is that converting the global offset to beats, instead of mixing beats and seconds, should keep the sync tighter when rate modding a song.

I'd like this to be tested by someone who has a better sense of timing accuracy than I do, but I think it's a better solution than going back to the old code (which seems to only work at the song level) or keeping the current code (Which seems to only work at the engine level).

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The if-else statements inside TimingData::GetElapsedTimeFromBeat(float fBeat) and TimingData::GetBeatAndBPSFromElapsedTime(GetBeatArgs& args) only execute the new code if m_fMusicRate is anything other than 1. They are necessary because stuff will break theme side if it's not done like this. Without the if-else statements, this will happen.

[teejusb]

I reallllllyy dont think we should be converting to beats for global offset stuff. If anything, we should be using only seconds since global offset is in seconds. Also if you only look at BPM and not stops i thinnnkk you're gonna get incorrect values on a lot of charts where the BPM is set to like 100000 and then counteracted with stops. This solution does not seem robust so I'd like to think about this some more.