Option to render maps front to back.

[BLamy]

So I have several layers. I give the user the option of what layer they want to appear on the top.

How it works now is the bottom layer is rendered, then the next layer is loaded, then the layer after that.
This means if I have 4 layers.

The user must wait for the 3 low-priority layers to render before it attempts to load the up-most layer.

This causes the user to have to wait for their data to be loaded. Instead, I would propose the option to allow the top layer to be rendered first then have the background layers rendered afterward.