add bevy_render::pass::ClearColor to prelude (#396)

It is common for users to want to set their clear color.

crates/bevy_render/src/lib.rs

@@ -20,6 +20,7 @@ pub mod prelude {
         draw::Draw,
         entity::*,
         mesh::{shape, Mesh},
+        pass::ClearColor,
         pipeline::RenderPipelines,
         shader::Shader,
         texture::Texture,