Arbitrary Alpha-Channel Masking

[ErikMcClure]

One of the requirements of Material Design is for certain animations or effects to be clipped precisely to a control, and a control could be multiple objects (like two rounded rectangles perpendicular to each other in a cross formation), which is impossible to write a general clip for. This needs to be solved using some sort of masking layer, potentially in combination with the layers introduced in #17 for rotational clipping support. This would function by simply setting the alpha channel of the layer to the inversion of the mask and then drawing things on it with the appropriate blend options.

[ErikMcClure]

This can now be done using blend states and layers.