Enable btHeightfieldShape in VehicleDemo, so it is tested (using heightfield128x128)
(converted raw data to .cpp so it doesn't need to be loaded from disk)
Some optimizations for btHeightfieldShape, thanks to Danny Chapman
 Quick check on AABB (was not done) and reduce calls to 'getVertex'


git-svn-id: http://bullet.googlecode.com/svn/trunk@2650 08e121b0-ae19-0410-a57b-3be3395fd4fd