set engine default to Sdl2

Always default to the best engine. If it fails,
all other supported engines are tried.
Merge pull request #4365 from hasufell/master

engine detection: fix order/redundancy
added contrail effects to sniper shots
Merge pull request #4453 from Mailaender/sniper-contrails

Added contrail effects to sniper shots
fix client join/leave race with master server ping
Generalise AttackMove to support anything with IMove and AutoTarget.

Also removes the unnecessary JustMove parameter.
Add AttackMove to C&C helicopters.

The other mods don't allow aircraft to AutoTarget.
English please
stich the black wreck together
merged the giant crater into one tile
merged the bottom of the big rock
also a dead worm
category resorting
identical to the right two tiles from 443

BLOXICE and BLOXTREE redundancy
remove building bibs to avoid confusion
randomized bricks
categorize everything
fixed a crash in the actor border rendering
Merge pull request #4461 from chrisforbes/master-ping-thread-safety

fixes #4459
Merge pull request #4464 from pchote/attackmove

Add AttackMove to C&C helicopters