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/AdlCollisionShape.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 COLLISION_SHAPE_H
+#define COLLISION_SHAPE_H
+
+#include "Stubs/AdlMath.h"
+#include "Stubs/AdlAabb.h"
+
+
+_MEM_CLASSALIGN16
+class CollisionShape
+{
+	public:
+		_MEM_ALIGNED_ALLOCATOR16;
+
+		enum Type
+		{
+			SHAPE_HEIGHT_FIELD,
+			SHAPE_CONVEX_HEIGHT_FIELD,
+			SHAPE_PLANE,
+			MAX_NUM_SHAPE_TYPES,
+		};
+
+		CollisionShape( Type type, float collisionMargin = 0.0025f ) : m_type( type ){ m_collisionMargin = collisionMargin; }
+		virtual ~CollisionShape(){}
+		virtual float queryDistance(const float4& p) const = 0;
+		virtual bool queryDistanceWithNormal(const float4& p, float4& normalOut) const = 0;
+
+	public:
+		Type m_type;
+		Aabb m_aabb;
+		float m_collisionMargin;
+};
+
+#endif