NML:Replace TTD sprites

Using a replace block, it is possible to replace TTD's built-in sprites. The syntax is as follows:

replace(<sprite-id>, <image-file>) {
	list of realsprites
}

The first parameter <sprite-id> indicates the sprite number of the first sprite to replace. Sprite numbers are equal to the sprite numbers in the base graphics file. The OpenGFX author overview can be used as a reference to look up these sprite numbers. The second parameter <image-file> specifies the image file that contains the new sprites. A series of real sprites has to be specified between the curly braces ({ and }). The first sprite in this list replaces the sprite with id sprite-id, the second replaces sprite-id + 1 and so on. Sprite templates can be used as well. For example:

 // Rail overlays for crossings
 replace (1005, "src/gfx/rails_overlays.png") {
 		[ 20,198, 40,21, -20,  5]
 		[ 71,198, 40,21, -20,  5]
 		[117,198, 40, 7, -20,  5]
 		[165,198, 40, 7, -20, 21]
 		[216,198, 12,19,  10,  5]
 		[248,198, 12,19, -24, 11]
 }