Several corrections to drawing routines

    * Improved custom alpha blender to make it properly work with non-opaque destinations;
    * Sprite alpha rendering style game option enables drawing sprites upon DrawingSurface, as well as drawing cursor hotspot sprite, using proper alpha blending.
    * Extended savedgame version which allows to serialize more game options; the extended format is used only if current game data is 3.3.0 or higher - this is done to keep forward compatibility with 3.2.1 engine.
    * Fixed text color for WFNFontRenderer;
    * Renamed few functions and parameters to better reflect their meaning.
    * More consistent use of "transparency/alpha" parameters in drawing functions.

    There's still a problem related to weird transparency value range for Characters and Objects. It works so that:
    0 = not transparent, 255 = invisible, 1..254 barely visible -to- mostly visible (like alpha).
    Basically, it could be used plainly as alpha value, if not the fully opaque and fully invisible states were switched.
    Some research must be done why this is so, and which convertions would be required to make it behave like a proper alpha value.