Overview should show extent of overview items, not all project layers

[jctull]

Currently, the overview defaults to the extent of the largest layer in the qgis project. Often, there is a layer that is beyond the extent of what you want to see in an overview. E.g., there may be a file that covers all of Europe, but you only want the overview map to show the outline of Germany, the area of primary interest. If the overview is only showing the outline of Germany, it should only cover the extents of Germany, not all of Europe.

[jctull]

Another example of how this works poorly: If I have a layer in the overview as a docked item below the layer legend, I can make the overview map shrink by adjusting the overview pane up and down (taller and shorter). With each subsequent movement of the window up and down, the overview map shrinks further. Again, the overview map should be pegged to the extent of the largest layer in the overview.