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/mc_id.h

@@ -290,6 +290,9 @@ enum mc_block_id {
     block_loom = 11367,
     block_stonecutter = 11368,
     block_grindstone = 11369,
+    block_mossy_stone_brick_stairs = 11370,
+    block_mossy_cobblestone_stairs = 11371,
+    block_mossy_stone_brick_wall = 11372,
 };
 
 typedef uint16_t mc_block_t;