Add MaskExtension

[felixpalmer]

For #6552

Change List

• Implementation of MaskExtension
• Example app for website

Tests & documentation in future PR, publishing this minimal WIP to gather feedback on API and to allow downstream implementation in CARTO for React

[felixpalmer]

@Pessimistress I followed the code in the other extension as well as the shadow module to implement this.

One thing I would appreciate you commenting on the the maths around the offset projection. I think what I have is close, but not quite right, and I see precision errors when zoomed in close. Relevant code: https://github.com/visgl/deck.gl/pull/6554/files#diff-e7aa5378c14ee05c714d147abc0b53e860e4f422f9fc85cae7d44bb4f3e9971aR56-R93

[coveralls]

Coverage decreased (-0.04%) to 80.26% when pulling 97be55a on felix/mask-extension-wip into 29373c6 on master.

[Pessimistress]

An extension is the wrong place to hold a FBO because it is run per layer. Mask(s) are global and shared and should be implemented in an Effect, which can be executed before and after any render and inject resources into all draw calls. Since we are adding a core operation prop it is not inappropriate to always load a MaskEffect by default. See how shadow FBOs are handled in https://github.com/visgl/deck.gl/blob/master/modules/core/src/effects/lighting/lighting-effect.js#L62