opl: Handle negative time division values.

A negative time division file indicates the MIDI file uses SMPTE time
rather than the normal time system. This is not supported yet, but for
the time being, return a sensible time division value that doesn't
cause the sound to stutter and the game to become unplayable.

This fixes #352, although the affected MIDIs do not yet play properly.
NEWS: Tweak text for next version.

Add reference to PHILOSOPHY file.
PHILOSOPHY: Add section on compatibility.

Add a section that discusses the different ways in which Chocolate
Doom aims for Vanilla compatibility (expanded version of the brief
description in README). Perform some minor rewording of the existing
text and add a reference to the philosophy file in README.