Add the GPU rigid body pipeline from https://github.com/erwincoumans/experiments as a Bullet 3.x preview for Bullet 2.80

Extras/RigidBodyGpuPipeline/dynamics/basic_demo/Stubs/AdlConstraint4.h

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+/*
+Copyright (c) 2012 Advanced Micro Devices, Inc.  
+
+This software is provided 'as-is', without any express or implied warranty.
+In no event will the authors be held liable for any damages arising from the use of this software.
+Permission is granted to anyone to use this software for any purpose, 
+including commercial applications, and to alter it and redistribute it freely, 
+subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
+*/
+//Originally written by Takahiro Harada
+
+
+#ifndef ADL_CONSTRAINT4_H
+#define ADL_CONSTRAINT4_H
+
+
+
+struct Constraint4
+		{
+			_MEM_ALIGNED_ALLOCATOR16;
+
+			float4 m_linear;
+			float4 m_worldPos[4];
+			float4 m_center;	//	friction
+			float m_jacCoeffInv[4];
+			float m_b[4];
+			float m_appliedRambdaDt[4];
+
+			float m_fJacCoeffInv[2];	//	friction
+			float m_fAppliedRambdaDt[2];	//	friction
+
+			u32 m_bodyA;
+			u32 m_bodyB;
+
+			u32 m_batchIdx;
+			u32 m_paddings[1];
+
+			__inline
+			void setFrictionCoeff(float value) { m_linear.w = value; }
+			__inline
+			float getFrictionCoeff() const { return m_linear.w; }
+		};
+
+#endif //ADL_CONSTRAINT4_H
+