Clean up syntax in DynamicKeyImageRenderer
Change how the default renderers work.

All default renderers now have their constructors set to internal.
Setting a new renderer on a RenderTarget that is a default renderer in
SharpBlade requires using the respective Set method on the RenderTarget.
Removed the confusing Images renderer property on DynamicKey.
Added more methods for managing images on DynamicKey to compensate for
removed Images property.
Minor code cleanup.
Code cleanup