Various improvements made to the airports:
- Climate-specific grass
- Use improved hangers from OpenGFX+ Airports
- Fixed missing pixels on one of the buildings
- Improved offsets for the terminal buildings
Credit for the hangers goes to zero.eight (https://www.tt-forums.net/viewtopic.php?p=959040#p959040); also, LordAzamath should probably get credit for his contributions to the small airport building (https://www.tt-forums.net/viewtopic.php?p=695162#p695162).
Preview sprites still need doing, will come back to those later.