MapContent subclass for use by DefaultRenderingExecutor

When asked to render Layers into separate graphics destinations, DefaultRenderingExecutor creates temp MapContent objects to hold each Layer and pass it to a renderer. Can't use the existing MapContent class for this because failing to call tempMapContent.dispose after rendering generates warnings, while calling dispose destroys the Layers.

Create a package-private sub-class, SingleLayerMapContent, which overrides dispose and finalize as empty methods. This can then be used to create temp instances.