More optimization, add constants file for eventual descructible scenery.

constants.lua

@@ -0,0 +1,45 @@
+-- unavoidable table lookups used by multiple files are stored here.
+-- nothing in here should
+-- be changed at runtime.
+--
+-- also has commented "fake constants" that are replaced by actual values, for
+-- reference.
+--
+-- see index_map.md for more "constant" values
+
+function init_constants()
+   -- block_size = 64
+   -- biome_size = 128
+
+   -- the indices here are sprite numbers.
+   object_interaction_map = {
+      -- tree
+      [4] = {
+         replacement = 14,
+         sfx = 11,
+         drop = 65
+      },
+
+      -- big mushroom
+      [8] = {
+         replacement = 16,
+         sfx = 12,
+         drop = 66
+      },
+
+      -- cactus w/ flower
+      [10] = {
+         replacement = 15,
+         sfx = 12,
+         drop = 68
+      },
+
+      -- cactus
+      [13] = {
+         replacement = 15,
+         sfx = 12,
+         drop = 67
+      }
+   }
+
+end