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

Extras/RigidBodyGpuPipeline/opencl/opengl_interop/btStopwatch.h

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+/*
+Stopwatch for timing and profiling for the Bullet Physics Library, http://bulletphysics.org
+Copyright (c) 2003-2011 Erwin Coumans
+
+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.
+*/
+
+#ifndef BT_STOPWATCH_H
+#define BT_STOPWATCH_H
+
+///The btStopwatch is a portable basic clock that measures real-time, use for profiling etc.
+class btStopwatch
+{
+public:
+	btStopwatch();
+
+	btStopwatch(const btStopwatch& other);
+	btStopwatch& operator=(const btStopwatch& other);
+
+	~btStopwatch();
+
+	/// Resets the initial reference time.
+	void reset();
+
+	/// Returns the time in ms since the last call to reset or since 
+	/// the btStopwatch was created.
+	float getTimeMilliseconds();
+
+	/// Returns the time in us since the last call to reset or since 
+	/// the Clock was created.
+	unsigned long int getTimeMicroseconds();
+private:
+	struct btStopwatchData* m_data;
+};
+
+
+#endif //BT_STOPWATCH_H