Improve visuals of the map preview on the mission briefing screen

[MrWabbit]

In source:/trunk/Code/missionbrief.mdl@106#L323

Before:

mapPreviewPane      = MapPreviewPane(ImportImage("sectormapopaquebkgndbmp",false),false,true);

After:

mapPreviewPane      = MapPreviewPane(ImportImageFromFile("mods/F7/media/sectormapopaquebkgnd.png", true),false,true);

[MrWabbit]

opaque background image

[MrWabbit]

edited missinbrief file

[MrWabbit]

If transparent minimap and inventory panes are going to be a cortui feature, better add this as well.

[Henrik Heimbuerger]

Please don't close the ticket before the change is committed to the repository. What is closed, is considered not requiring any additional work.

[MrWabbit]

I'll take care of this when i make the inventory and minimap mods.