Fix FrameEvent: hold mutex during draw + e.Frame() per architecture doc §8

cmd/pad/main.go

@@ -81,13 +81,12 @@ func run(w *app.Window) error {
 				prevScale = newScale
 			}
 
-			// Acquire mutex, read frame, draw, release mutex
+			// Acquire mutex, read frame, draw, e.Frame(), release mutex
 			mu.Lock()
 			currentElems := elems
-			mu.Unlock()
-
 			renderer.Draw(gtx, currentElems)
 			e.Frame(&ops)
+			mu.Unlock()
 		}
 	}
 }