Support base layers in RenderDynamicOverlay

[jng]

Currently if we want to render an image of a map with any tiled base layers at any arbitrary scale, we have to use the RenderMap API.

The problem with the RenderMap API, is that the background color is pre-filled into the image making it fully opaque and un-suitable for overlaying on top of another image.

The problem with the RenderDynamicOverlay API (which is normally the API we would use in this case) is that tiled base layers are omitted during rendering.

Attached is a patch that introduced a new behavior flag (RenderBaseLayers) for the RenderDynamicOverlay API, and a modification to RenderDynamicOverlay to include such tiled base layers if the new RenderBaseLayers is OR'ed into the behavior parameter.

[jng]

Fixed r7109