FR: Add improved contrast options for HUD

Some HUD elements become hard to see against certain the background of certain missions, or against certain objects, even with the "High Contrast" mode.
The simplest solution would be to add a stroke or a "drop shadow" in contrasting color to the HUD elements.
This would probably need to be a command line option.
The color of the stroke or shadow could either be set in the command line as well, or default to [the most contrasting color to the HUD element's color](https://gamedev.stackexchange.com/questions/38536/given-a-rgb-color-x-how-to-find-the-most-contrasting-color-y).

Doing it as drop shadow would probably not be difficult from what I can tell of HUD code.
Doing it as a stroke would probably require applying a shader to each HUD element.