Linkgraph: Scroll overlay pixel cache instead of clearing it on scroll

Reduce cost of preparing/drawing cache
Improve missing station checks in RefreshDrawCache
Remove need for checks in DrawLinks
Don't rebase overlay cache coordinate to screen in viewport map mode

src/viewport_func.h

@@ -103,7 +103,6 @@ bool ScrollWindowTo(int x, int y, int z, Window *w, bool instant = false);
 void UpdateActiveScrollingViewport(Window *w);
 
 void RebuildViewportOverlay(Window *w, bool incremental);
-bool IsViewportOverlayOutsideCachedRegion(Window *w);
 
 bool ScrollMainWindowToTile(TileIndex tile, bool instant = false);
 bool ScrollMainWindowTo(int x, int y, int z = -1, bool instant = false);