slideshow/glTF: make sure parent window does not cover the OpenGL window

Change-Id: I0da3978cefeb3d145f7411cc1c83a87d981cac62
OGLWindow: fix key capturing issue

Sometimes the event handler parent captures the mouse
moce which should grab the focus for key event handling.

Change-Id: Ida1022968543059154de824fc3d026c5cecba7b3