Tile layers with parallax are not properly rendered

[k2d222]

See BlueChaos map for an obvious example. related to #18.

[ChillerDragon]

Guess that is related to the linters warning

https://github.com/k2d222/twwe/runs/6578175033?check_suite_focus=true

/home/runner/work/twwe/twwe/client/src/gl/renderGroup.ts:37:13
Hint: 'offX' is declared but its value is never read. 
    const mv = mat4.create()
    const { offX, offY, paraX, paraY } = this.group
    const { x1, x2, y1, y2 } = viewport.screen()

/home/runner/work/twwe/twwe/client/src/gl/renderGroup.ts:37:19
Hint: 'offY' is declared but its value is never read. 
    const mv = mat4.create()
    const { offX, offY, paraX, paraY } = this.group
    const { x1, x2, y1, y2 } = viewport.screen()