Use WebGL instead of Canvas2D for rendering

[Demonessica]

Is your feature request related to a problem? Please describe.
Canvas2D is generally slow, especially on Firefox, and I've encountered performance issues where the putImageData() and drawImage() calls take significantly longer than they generally should. (see #440)

Describe the solution you'd like
Migrating to a WebGL hardware-based rendering pipeline may improve performance and optionally allow for more advanced 3D graphics if called for in the future.

Describe alternatives you've considered
Another approach is to have the individual canvases the layers render to be directly visible in the viewport instead of being copied to one main canvas, using CSS transformations for pan/zoom. This should cut down on canvas draw operations and can use hardware acceleration for CSS rendering if enabled.

[evanpelle]

I think migration can happen in two stages:

1st stage: the top level canvases like unit, fx, structures can be converted to a pixi CanvasRenderer and rendered on top of the existing canvas
2nd stage: the territory layer & terrain layer can be migrated to webgl.

[wraith4081]

could you assign it to me?