add raycast accelerator for btHeightfieldTerrainShape, thanks to Marc Zylann, see https://github.com/bulletphysics/bullet3/pull/2062

it can be disabled by setting the flag cb.m_flags |= btTriangleRaycastCallback::kF_DisableHeightfieldAccelerator;
acceleration is disabled for z axis up.
add btHeightfieldTerrainShape example to example browser

examples/ExampleBrowser/premake4.lua

@@ -169,6 +169,7 @@ project "App_BulletExampleBrowser"
 		"../Collision/Internal/*",
 		"../Benchmarks/*",
 		"../MultiThreadedDemo/*",
+		"../Heightfield/HeightfieldExample.*",
 		"../CommonInterfaces/*.h",
 		"../ForkLift/ForkLiftDemo.*",
 		"../Importers/**",