improve (fix?) custom map rendering [possible bug]

[Mis012]

There are two issues I see with the current approach to custom map rendering:

1. some features on of the non-custom map (e.g any text) are hidden when a custom map is being shown, even if the trasparency is set to near 100% and most features of the non-custom map are clearly visible. I'm not sure if this is expected behavior or a bug, but I'm leaning towards the latter
2. it would be really handy if there were other blending modes available, for example the multiply blending method is vastly superior for most usecases (and in my experience would be an easy fix for Use custom map layer as background #103)

[andreynovikov]

1. There is a switch "Hide map objects", turn it off to see them.