tweaks...
Menu :
1. only draw the title image when player is not yet in a game
2. prevent showing the menu at startup [ this is a WIP.... ]
3. moved some code around
Menu : got it working properly when player is not in a game, pressing a key
brings up the menu and pressing ESCAPE goes back to a bare title screen.
Game / inventory :
1. define player fields we will use (i_keys, i_armor etc)
2. added a 'mana' field for players
3. updated encode/decode logic of spawnparms for these
Screen lib : added SCR_DrawBackground() which will make the image fill
the vertical space of the screen and be centered horizontally.

Menu : use this for drawing the title background.
Data : updated defaults.cfg (e.g. disable fullscreen by default).
Menu : darken screen when showing a menu.