updated viewport clearing

This change is needed in order to make use of the new introduced
set_clear_mode() function (I think the choosen enum value is ok since
its needed only once).
Updated in the following demos:
- 2D_in_3D
- 3D_in_2D
- screen_capture

viewport/2d_in_3d/2D_in_3D.gd

@@ -3,7 +3,7 @@ extends Spatial
 func _ready():
   # Get the viewport and clear it
   var viewport = get_node("Viewport")
-  viewport.clear()
+  viewport.set_clear_mode(Viewport.CLEAR_MODE_ONLY_NEXT_FRAME)
   # Let two frames pass to make sure the vieport's is captured
   yield(get_tree(), "idle_frame")
   yield(get_tree(), "idle_frame")

viewport/3d_in_2d/3D_in_2D.gd

@@ -16,7 +16,7 @@ func _ready():
   viewport_sprite = get_node("Viewport_Sprite")
   
   # Assign the sprite's texture to the viewport texture
-  viewport.clear()
+  viewport.set_clear_mode(Viewport.CLEAR_MODE_ONLY_NEXT_FRAME)
   # Let two frames pass to make sure the screen was captured
   yield(get_tree(), "idle_frame")
   yield(get_tree(), "idle_frame")

viewport/screen_capture/screen_capture.gd

@@ -7,7 +7,7 @@ func _ready():
 
 
 func _on_button_pressed():
-  get_viewport().clear()
+  get_viewport().set_clear_mode(Viewport.CLEAR_MODE_ONLY_NEXT_FRAME)
   # Let two frames pass to make sure the screen was captured
   yield(get_tree(), "idle_frame")
   yield(get_tree(), "idle_frame")