Fix hellfires spawning only at left mount
Make animations more flexible

* Allow adding more frames after animation is constructed
* Allow specifying list of individual frames to add
Tweak explosions

* Use original explosion animation sequences (use new Animation features)
* Use original durations
* Rename explosions to hydra/hellfire for clearness
Use more Desert Strikely ground color
Add assert on unimplemented Explosion::GetLifetime
Log which file we're going to display
Support decoding non-transparent sprites
Support decoding PICTURE section
Extend bullet explosions
Add large explosion (not used yet)
Add assert on unimplemented Explosion render