Tune particles sizes

Coin burst particle size had gotten out of sync with goal particles size.
Also, particle size now takes into account HMD mode.

As point sprites, particle sizes are specified in real on-screen pixels
instead of in world units (meters). Size is determined as a fraction of
screen height so that they remain consistent through resolution changes.
However, HMD mode uses a high-resolution off-screen buffer, and that
buffer has an 8:9 ratio instead of 16:9, so the particle size calculation
has to be special-cased.