New interface design of WebGLEngine

[eyworldwide]

Is your feature request related to a problem? Please describe.

The parameters of WebGLEngine are strange, especially I have to pass a undefined value to the second parameter when I want to set the canvas's property:

new WebGLEngine('canvasId', undefined, {alpha: true})

Describe the solution you'd like

Physics is a system of engine, maybe we should add system concept to engine, the API is like this:

webglEngine.addSystem(PhysxSystem);

The interface of System is more scalable, and we can add more systems in the future.

Design: @GuoLei1990

PR: @GuoLei1990

PR reviewers: @GuoLei1990 , @yangfengzzz , @eyworldwide

[GuoLei1990]

How to say, the physical parameters here are not the physical system itself. The second is what kind of physical backend should be used.

But the problem you described is worth solving.