Simplify draw state

[bvssvni]

Instead of converting from one draw state API to another, an idea is to use a simplified draw state that cover the use cases we have today.

pub struct DrawState {
    pub blend: Blend,
    pub stencil: Option<Stencil>,
    pub scissor: Option<[u32; 4]>,
}

impl Default for DrawState {
    fn default() -> Self {
        DrawState {
            blend: Blend::Alpha,
            stencil: None,
            scissor: None,
        }
    }
}

pub enum Blend {
    Alpha,
    Add,
    Multiply,
    Invert,
}

pub enum Stencil {
    Clip(u8),
    Inside(u8),
    Outside(u8),
}

[bvssvni]

Since additive blending gives different effect than adding color components for semi-transparent colors (see #1001), I suggest that we remove Add and Multiply to avoid confusion. Instead we could add a variant that lets you control the equations in more detail.

[bvssvni]

There seem to be several blending techniques with different benefits/drawbacks:

• Simple: A linear equation with two terms as implemented in fixed hardware pipelines
• Pre-multiplied alpha: Simple + a step in the fragment shader that preprocesses the colors
• Porter/Duff: A complete algebra for blending that requires an additional term with the product of both source and destination alpha channel

[bvssvni]

I am considering implementing this for the new Gfx design. See #1030.