applies several visual tweaks

full changelog:
- 2nd frame of Yak shp no longer uses greyscale instead of remap for the tail fins.
- Mobile Gap Generator finally has a shadow.
- Medium Tank barrel length increased by ~1-2 pixels (depends on angle).
- Light Tank barrel significantly downsized to match its low damage per shot.
- airfield now uses all 8 frames for flag anim and active anim.
- airfield house/tower empty pixels filled with shadow..
- Gap Generator now has some shadow.
- FRAG1 and FBALL1 explosions are now a bit more centered.
- wood crate received a shadow and was converted to the correct palette.
- silver crate now has a shadow, too.
- fixed some minor errors on Harisson's silo and added some shadow.
- Construction Yard got some shadow to better fit in line with other buildings.
- Pistol now uses single piff instead of multiple piffs.
- 50cal added to sequences.
- Light tank now uses 50cal instead of 120mm bullet.