resolve some naming conflicts between Bullet 3.x and Bullet 2.x

add BasicGpuDemo that bridges Bullet 2.x and Bullet 3.x (still preliminary/crashing/instable)

src/Bullet3Common/b3QuadWord.h

@@ -52,6 +52,7 @@ public:
 #else //__CELLOS_LV2__ __SPU__
 
 #if defined(B3_USE_SSE) || defined(B3_USE_NEON) 
+public:
 	union {
 		b3SimdFloat4 mVec128;
 		b3Scalar	m_floats[4];