Fixed nether rendering crash

The nether rendering mode was not properly handling x and z values
outside of the range 0 to 15.  This was causing an out-of-bounds
segfault when accessing the remove_block array.

Fixes #881
Fixes #851
Fixes #852

overviewer_core/src/primitives/nether.h

@@ -22,9 +22,11 @@
 #define DEPTH 16
 #define HEIGHT 256
 
+// add two to these because the primative functions should expect to
+// deal with x and z values of -1 and 16
 typedef struct {
     int walked_chunk;
 
-    int remove_block[WIDTH][HEIGHT][DEPTH];
+    int remove_block[WIDTH+2][HEIGHT][DEPTH+2];
     
 } RenderPrimitiveNether;