Display a Dæmon logo at startup, before everything else can be displayed and before the game can do anything

[illwieckz]

When the engine starts, there is a certain amount of time between the moment the window is created and the game starts to display something. Basically, the game has to load the VFS and run the UI code to display something, but before that the displayed window is empty.

To avoid garbage being displayed at this early stage (see #373), PR #374 just fill the window with a neutral-like dark grey color as early as possible.

The next step to make things look good and more welcoming would be to display a simple logo.

The logo would have to be displayed as early as possible, so before VFS is loaded. Because of that the logo would be unique whatever the game, and would have to be embedded in engine for easy use.

To make it neutral regarding games running on Dæmon, we may imagine a Dæmon logo, about the engine itself. Also that would take part in building reputation about the engine itself, something like “Powered by Dæmon”.

I made a simple mockup, reusing the “Unvanquished Development” icon and a random font:

[Edit: as showcased by this mockup, a monochrome logo would be perfect]