Fix: Poor contrast in cargo dest flow legend window cargo labels.

Select foreground colour depending on the brightness of the background.

Previously all cargo labels were rendered using black text, even
the background cargo colour was dark/black.
As an example: FIRS coal was black text on a black background.

src/gfx_func.h

@@ -187,7 +187,7 @@ int GetCharacterHeight(FontSize size);
 
 extern DrawPixelInfo *_cur_dpi;
 
-TextColour GetContrastColour(uint8 background);
+TextColour GetContrastColour(uint8 background, uint8 threshold = 128);
 
 /**
  * All 16 colour gradients