Add brushes as resources to the scene manager

[CryZe]

As it turns out, creating brushes is actually quite expensive with a lot of rendering backends. So we definitely want to only create them once and then just attach them to the individual entities.

[Hurricane996]

I think we should globally cache solid color shaders because there's likely to be a lot of them that are mostly identical. The other shaders all depend on width and height so tbey should probably belong to a local cache, but for the color shaders I see no harm in just having a lazy_static lru cache somewhere with a reasonable size.

[Hurricane996]

although checking again it appears the solid color shaders are also by far the least computationally expensive, to the point where caching it is probably not even worth it.