Improving BlendState/RasterizerState/DepthStencilState.ToString()

Fix: BlendState.ToString() to not display "Opaque" for all custom states.
Improvement: BlendState.ToString() now displays the short type name instead of the namespace-qualified type name.
Improvement: Added ToString() to DepthStencilState and RasterizerState, mirroring the implementation of BlendState.ToString().

Note that these methods are primarily for improving workflow while debugging, since these objects are displayed in the watch window by the evaluation of their ToString() method.