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/CustomConvexShape.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 Erwin Coumans
+
+#ifndef CUSTOM_CONVEX_SHAPE_H
+#define CUSTOM_CONVEX_SHAPE_H
+
+#include "BulletCollision/CollisionShapes/btConvexHullShape.h"
+
+class CustomConvexShape  : public btConvexHullShape
+{
+	public:
+		
+		class ConvexHeightField* m_ConvexHeightField;
+
+		int m_acceleratedCompanionShapeIndex;
+
+		CustomConvexShape(const btScalar* points,int numPoints,int stride);
+		virtual ~CustomConvexShape();
+		
+};
+
+#endif //CUSTOM_CONVEX_SHAPE_H
+