Engine: extended "--gfxfilter" command-line argument

This makes "--gfxfilter" argument take filter scaling as a second parameter.
Engine: limit game scaling with fixed filter scaling factor (if any)
Engine: corrected some default configuration values

The defaults for graphics mode is video mode keeping current display aspect ratio, standard scaling filter and centered game frame (even proportional stretch does not always look enough for some games, depending on display resolution, so we are leaving this up to user if he/she would want to stretch game to the window).
Engine: made mouse managing functions take game viewport into account

Although the display mode black borders are no longer part of the game business logic, there are still extra borders created by game for letterbox-by-design mode, and when the room is smaller than the game size. This commit makes mouse coordinate translation take extra viewport offset into account.
Script API: corrected System.ScreenWidth and ScreenHeight properties

It appears that in AGS these do not tell the real screen (window) size, but *unscaled* game frame size.