Aircraft fixes, changes

Aircraft:
-increased alpha it is drawn with
-changed scaling function
+now linear, but offset from zero
-fixed plane position appearing offset
-will now automatically calculate the origin of the plane image
!so long as it's centred
possible bugfix?

Bugfix for rare case where planes flicker between airports?
Merge branch 'master' of https://github.com/SEPRGroup/SEPR4Project