Allow update of drawing items

[slvrfn]

A user can now have access to the drawing items, and can provide a short list of items that need to be re-calculated. Intended to be used to replace shapes in-place in the items list.

One example is re-coloring only certain drawing items, all items do not need to be re-added/re-calculated.

Moved transformColor to a static context that can be used outside the library when the user is creating items.

[FabianTerhorst]

Im going to merge #16 first. Do you want to take a look to see how much the performance improved?Mostly just the amount of allocations is reduced. isPointInPoly is now the slowest calculation when having sort activated.

[slvrfn]

I definitely believe that about isPointInPoly. There can be a whole lot of edges to check when drawing. Do you have any ideas to improve that method?

I'll definitely take a look at the other performance improvements!

[slvrfn]

I have added some commits which include the ability to convert to/from Isometric and View coordinate systems.