Use magic number for bitmap, screen and sprite, make script resource management easier.
Remove script function getpixel, use getgfxproperty instead.
* getgfxproperty(handle, propertyname, ...)
- handle can be sprite or screen
- propertyname: width, height, srcwidth, srcheight, centerx, centery, pixel...
- for pixel, x, y must follow
- other properties such offsetx, offsety have not been added yet.
