Replaces pixels with OldPixel in the given rectangle by NewPixel.

public procedure ReplaceColor(X: LongInt; 
                              Y: LongInt; 
                              Width: LongInt; 
                              Height: LongInt; 
                              OldColor: Pointer; 
                              NewColor: Pointer);

OldPixel and NewPixel should point to the pixels in the same format as the given image is in.