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/block_class.h

@@ -46,4 +46,7 @@ extern const size_t block_class_ancil_len;
 extern const mc_block_t block_class_alt_height[];
 extern const size_t block_class_alt_height_len;
 
+extern const mc_block_t block_class_wall[];
+extern const size_t block_class_wall_len;
+
 #endif