joy_demo: Don't start preview if alpha is 0.

Running the preview pipeline and having DispmanX compose with
alpha blending adds to the total workload, potentially increasing
thermal output. If it's not gonna be visible, don't do it.

Change-Id: Icd10989f217eadddaa3fefe1c1d01ccfe409e436

src/examples/vision/joy/joy_detection_demo.py

@@ -263,7 +263,11 @@ def take_photo():
                     photographer.shoot(camera)
 
                 # Blend the preview layer with the alpha value from the flags.
-                camera.start_preview(alpha=preview_alpha)
+                if preview_alpha > 0:
+                    logger.info('Starting preview with alpha %d', preview_alpha)
+                    camera.start_preview(alpha=preview_alpha)
+                else:
+                    logger.info('Not starting preview, alpha 0')
 
                 button = Button(23)
                 button.when_pressed = take_photo