Syncing was already incomplete (e.g. decoding takes more than one
frame and delays can happen in this case). Giving up syncing audio
timer and game timer should not affect the game experience so much.
Instead, clock implementation will be much simpler.
Before this change, the audio is suspended when the game stops for
1/12[s]. However, as game often stops for more than 1/12[s]
especially on mobiles, this implemntation caused some audio
glitches.
This change fixes this problem by re-implementing suspending/
resumeing audio by detecting the window is active/focused or not.
nosync package for GopherJS is problematic since nosync's mutex
assumes that Lock call is never duplicated, which is not true in
actual applications.
See also #603.
