Add support for scissor and viewport settings

[vanruesc]

Description

Add support for scissor/viewport settings to Pass and RenderPipeline.

Related: #386

Tasks

• [ ] Add scissor and viewport settings to Pass.
• [ ] Consider also adding scissor and viewport settings to RenderPipeline.
• [ ] Check if such settings also make sense elsewhere.
• [ ] Add a scissor-viewport demo.

Implementation Details

• Use the scissor, scissorTest and viewport settings on WebGLRenderTarget.