disabled casting shadows on pixel perfect lights in combination with cas...

Hi @collinhover,

i played around with a fix regarding to Issue #128. The simplest way in my opinion, is to avoid adding opaque entities if ```pixelPerfect``` and ```castsShadowsMovable``` is true. All other solutions that i have tried, had the side effect off disabling shadow casting based on the collision map as well.

I also altered the docs regarding to that.