Revert "reverted the last changes on the `globalAlpha` function"

This reverts commit df9824062b5d704d1ecd8ea0305805edbf0c17a4.
properly fixed the issue with `globalAlpha`
Fix collision detection between `me.Ellipse` (circle) and `me.Line` shapes

- `me.Line` is a special case of `me.Polygon` that has only a single edge.
- This patch does not attempt to check the "next" or "previous" edges of the `me.Line`
[#610] Fix me.Container positioning

- Reverts the broken inViewport test; use the object geometry instead of object bounding box
- Fixes the me.Entity geometry for world shapes created with Tiled PolyLine objects

Revert "[#569] Fix the disappearing platforms"

This reverts commit 104b6f45a7c366a662582a796a922fde2ec51ad0.

Conflicts:
    src/renderable/container.js
Update CHANGELOG
Merge branch '2.0.x'