initial version of 'nether' option for normal render modes

based on eminence's gist https://gist.github.com/92b6dac4c2ed3cdcff10

overviewer_core/src/rendermodes.h

@@ -136,6 +136,7 @@ typedef struct {
     unsigned int min_depth;
     unsigned int max_depth;
     int height_fading;
+    int nether;
 } RenderModeNormal;
 extern RenderModeInterface rendermode_normal;