Create larger OpenGL 3 render buffers

When rendering with multiple tiles, IceT's floating viewport only works if the geometry fits within the rendering buffer. This is much more likely if the rendering buffer is bigger.

The OpenGL 3 rendering layer uses textures for an offscreen rendering buffer. We can make this buffer as large as we want (given constraints of the rendering system). As such, make the rendering buffer the same size as the whole display (or as large as possible if that is not possible).