added graphics config option for level-specific colors for BD engine

added graphics config option for level-specific colors for BD engine

This adds a config option "bd_game_graphics_color_template" for the
custom artwork config file "graphicsinfo.conf" to specify a template
image file used to create level-specific game graphics colors for
native Boulder Dash level files (to be used for all game elements
for both in-game graphics and main menu preview graphics), while the
normal game element image config options specify a default image file
with static/fixed colors to be used in the level editor (that is, all
levels in the level editor use the same colors).

This implementation only works correctly for all game element graphics
if all BD style game graphics are defined in one single image file, so
the newly defined template image file must have the same image size
and layout of game graphics as the default image file (which must also
be a single image file containing game graphics for all game elements).