`ShaderDefVal` can be optimized by using `Cow<'static, str>`

[beicause]

ShaderDefVal uses String:

pub enum ShaderDefVal {
    Bool(String, bool),
    Int(String, i32),
    UInt(String, u32),
}

But most ShaderDefVals are &'static str. Replace the String with Cow<'static, str> can reduce heap allocations.