Use associated type for draw state

[bvssvni]

Blending is not usually required for widgets, and using a custom draw state means overhead. Perhaps we could use the native draw state?

• A software rasterizer might want full Duff/Porter blending
• A GPU rasterizer might want pre-multiplied alpha with a simple blend equation

Some features, such as rectangle clipping and stencil buffer, could be handled through a trait.

[bvssvni]

Gfx might change to PSO, which stores some extra stuff in draw state. This might be problematic to map to Glium's draw state, but not sure yet. See gfx-rs/gfx#815