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/iterate.c

@@ -390,7 +390,7 @@ generate_pseudo_data(RenderState* state, uint16_t ancilData) {
             }
         }
         return data;
-    } else if (state->block == block_cobblestone_wall) {
+    } else if (block_class_is_subset(state->block, block_class_wall, block_class_wall_len)) {
         /* check for walls and add one bit with the type of wall (mossy or cobblestone)*/
         if (ancilData == 0x1) {
             return check_adjacent_blocks(state, x, y, z, state->block) | 0x10;