Revert "ebiten: Skip calling Draw when the update count is 0"

This reverts commit 4d545b9ad8.

Reason: Restricting calls of Draw was counter-intuitive for users.
See the discussion at #1486

uicontext.go

@@ -190,12 +190,13 @@ func (c *uiContext) update(updateCount int) error {
 		uiDriver().ResetForFrame()
 	}
 
-	if updateCount > 0 {
+	// Even though updateCount == 0, the offscreen is cleared and Draw is called.
+	// Draw should not update the game state and then the screen should not be updated without Update, but
+	// users might want to process something at Draw with the time intervals of FPS.
 	if IsScreenClearedEveryFrame() {
 		c.offscreen.Clear()
 	}
 	c.game.Draw(c.offscreen)
-	}
 
 	// This clear is needed for fullscreen mode or some mobile platforms (#622).
 	c.screen.Clear()