Provide actual bounds to Shader primitives

... and allow for proper translation and scissoring.

wgpu/src/layer/pipeline.rs

@@ -0,0 +1,17 @@
+use crate::core::Rectangle;
+use crate::primitive::pipeline::Primitive;
+
+use std::sync::Arc;
+
+#[derive(Clone, Debug)]
+/// A custom primitive which can be used to render primitives associated with a custom pipeline.
+pub struct Pipeline {
+    /// The bounds of the [`Pipeline`].
+    pub bounds: Rectangle,
+
+    /// The viewport of the [`Pipeline`].
+    pub viewport: Rectangle,
+
+    /// The [`Primitive`] to render.
+    pub primitive: Arc<dyn Primitive>,
+}