fixed bug with not recognizing ".mode_loop: false" for music

Before, music was always played in loop mode, even if it was defined
with option "<music>.mode_loop: false".

Now, disabling loop mode for music in config files will indeed cause
music to be played only once.

Note: This commit changed the behaviour of the existing function
"PlayMusic()" (which always played music in loop mode before), which
will now play music only once, while a new function "PlayMusicLoop()"
was added, which always plays music in loop mode.