Implement MotionBlurEffect

[vanruesc]

Description

Implement a MotionBlurEffect.

Related: #248

References:

• https://github.com/gkjohnson/threejs-sandbox/tree/8ebf61b0a36d188ccf50f31d31dacc44319e3986/motionBlurPass

Tasks

• Create MotionBlurEffect.
• Create motion-blur demo.
• Add unit test.

Implementation Details

• Should be done after GBuffer.VELOCITY is implemented.
• Use G-Buffer components (depth, velocity).