Added set_scissor_rect to tracked render pass. (#3320)

# Objective And Solution
Add `set_scissor_rect` from wgpu-rs to the `TrackedRenderPass`. wgpu documentation can be found here:
https://docs.rs/wgpu/latest/wgpu/struct.RenderPass.html#method.set_scissor_rect

The reason for adding this is to cull fragments that are outside of the given rect. For my purposes this is extremely useful for UI.

crates/bevy_render/src/render_phase/draw_state.rs

@@ -228,4 +228,11 @@ impl<'a> TrackedRenderPass<'a> {
 
         self.pass.set_stencil_reference(reference);
     }
+
+    /// Sets the scissor region.
+    /// Subsequent draw calls will discard any fragments that fall outside this region.
+    pub fn set_scissor_rect(&mut self, x: u32, y: u32, width: u32, height: u32) {
+        debug!("set_scissor_rect: {} {} {} {}", x, y, width, height);
+        self.pass.set_scissor_rect(x, y, width, height);
+    }
 }