Implement some mossy blocks

This adds:
- mossy stone brick stairs
- mossy cobblestone stairs
- mossy stone brick walls

We also add another block class for walls, and while we're at it,
clean up the stairs texture function to not have a huge sprawling
elif mess for loading textures, but instead to a cheeky dictionary
lookup.

In case you're wondering, yes I am just as disgusted by this code as
you are.

overviewer_core/src/overviewer.h

@@ -31,7 +31,7 @@
 
 // increment this value if you've made a change to the c extesion
 // and want to force users to rebuild
-#define OVERVIEWER_EXTENSION_VERSION 69
+#define OVERVIEWER_EXTENSION_VERSION 70
 
 #include <stdbool.h>
 #include <stdint.h>