FR: Omnisprite option for weapon projectiles

Laser bitmaps behave like _directional billboards_, rotating to face the camera only around a single axis -- the direction of movement. However, it would often be useful to have bitmaps attached to projectiles which behave like _omnisprites_, rotating around arbitrary axes to face the camera and having a constant upward orientation with respect to it. This is especially useful for effects like lens flares.

Current options for doing this are limited and hacky. Therefore, I request a dedicated feature for attaching static or animated omnisprite bitmaps to weapons, completely separate from (and therefore usable in addition to) the projectile's model or laser bitmap. The ability to control the size of the bitmap would be desirable for obvious reasons, as would the ability to adjust its location relative to the weapon's center.

Proposed syntax:

$Omnisprite bitmap: _string_
+Omnisprite center: _vector_
+Omnisprite radius: _float_