Optimise the reflection map buffer for MeshRenderer

[fallahn]

Currently the Scene object renders the two farthest layers to a downsampled buffer, which is then used by the MeshRenderer as a reflection map. This is sub-optimal, as the MeshRenderer may not be using any materials which require a reflection map, or the Scene may not contain drawables on the rear layers - making any buffering moot. Suggestion is to create a specialised componented for rendering reflection maps so that the map is only drawn if it explicitly requested. This means the MeshRenderer can also be made more flexible by requiring the texture to be used as the reflection map to be explicitly stated. By default it would have a reflection map component target passed to it, but for situations where the far layers of the scene don't make sense (for example in a top down view the reflection would be that of the sky behind the camera, not the ground in front of it) custom reflection maps can be supplied to the MeshRenderer.