Add a subtle glow animation to standing Verlissh machine units
Add BOSS_EXPLOSION_FRAME and BOSS_BURST_FRAME animation frame prototypes

Both are taken from the death animation WML for the UtBS Weakened
Central Body (Central Body2) unit, from which the Verlissh Matrix Core
is derived. The exact WML was taken from the 1.10 version, simplified,
and refactored. It may be useful as a generic animation frame prototype
later.

This version also includes auto_hflip and auto_vflip parameters that
allow doing the same things as the original without quadruplicating code
for different unit facing directions.
Add a BOSS_*_FRAME-based death animation for the Verlissh Matrix Core

This provides essentially the same animation used in UtBS (Wesnoth
1.10.x) by the Weakened Central Body (Central Body2) unit type, from
which the Verlissh Matrix Core is derived.