Added Virtual Display capability

The latest release adds the ability to define a virtual display of any size. This display can use any of the text and drawing features, then render portions of the display across multiple physical displays. For example, you can place 2 128x64 OLEDs side by side and treat them like a 256x64 display. The virtual display will only update the memory and copying the data to the physical displays must be explicitly defined in your code using obdDumpWindow(). See the virtual_display example sketch for more info.