Fix bufferSource.loop = true on extreme edge case

[Taym95]

Fixes: servo/servo#41073

Previously:
the moment we set bufferSource.loop = true, the node hit the early “extreme edge case” check, decided “I refuse to output”, triggered onended, and returned silence. So the loop never got a chance to run.

Now:

• Early bailout is removed(no idea why was there in first place).
• The per tick offset is guaranteed to be a sensible value with respect to the loop range (wrapped if extreme).
• The normal mixing code runs, updating pos each tick and respecting the loop region.

[jdm]

Makes sense! Thanks!