rpg: compact mini tilesets via scripts/pack_tileset.py

scripts/pack_tileset.py reads a v1 map JSON, scans every layer.data
for unique non-zero tile ids, slices those cells out of the
referenced source PNG, and packs them into a Cols-wide grid (default
8). Emits a new packed PNG and rewrites the JSON's data arrays +
tilesets[0].src so ids renumber from 1.

Run for the RPG demo:
- examples/rpg/Overworld_mini.png  (8x1, 7 tiles: grass, water,
  rock, door, tree, sand, dirt)
- examples/rpg/cave_mini.png       (8x1, 4 tiles: floor, wall,
  stairs, water)

JSONs updated with the new ids (1..7 / 1..4) and src pointing at the
mini PNGs. rpg.bas SPRITE LOAD lines swap to the packed sheets.

Why: the IDE map editor's 8x4 tile picker can show the entire used
set without paging through 1440-cell Overworld.png, and the tiny
sheets are easy to redraw fresh — old mismatched picks like the
'least bad' rock/sand can be replaced one-for-one without touching
the maps. Original Overworld.png / cave.png stay in the tree as
authoring source. Shooter wasn't packed (already only 3 ids).

Author: omiq

examples/rpg/level1_cave.json

@@ -13,15 +13,15 @@
   "tilesets": [
     {
       "id": "cave",
-      "src": "cave.png",
+      "src": "cave_mini.png",
       "cellW": 16,
       "cellH": 16,
       "tiles": {
-        "162": {
+        "2": {
           "solid": true,
           "kind": "wall"
         },
-        "282": {
+        "4": {
           "solid": true,
           "kind": "water"
         }
@@ -34,27 +34,27 @@
       "type": "tiles",
       "tilesetId": "cave",
       "data": [
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
         1,
         1,
         1,
@@ -73,8 +73,8 @@
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
@@ -93,8 +93,8 @@
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
@@ -113,8 +113,8 @@
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
@@ -126,15 +126,15 @@
         1,
         1,
         1,
-        282,
-        282,
-        282,
-        282,
+        4,
+        4,
+        4,
+        4,
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
@@ -146,35 +146,35 @@
         1,
         1,
         1,
-        282,
-        282,
-        282,
-        282,
+        4,
+        4,
+        4,
+        4,
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
         1,
         1,
-        162,
-        162,
-        162,
-        162,
+        2,
+        2,
+        2,
+        2,
         1,
         1,
-        282,
-        282,
-        282,
-        282,
+        4,
+        4,
+        4,
+        4,
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
@@ -193,8 +193,8 @@
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
@@ -213,8 +213,8 @@
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
@@ -233,8 +233,8 @@
         1,
         1,
         1,
-        162,
-        162,
+        2,
+        2,
         1,
         1,
         1,
@@ -244,7 +244,7 @@
         1,
         1,
         1,
-        168,
+        3,
         1,
         1,
         1,
@@ -253,27 +253,27 @@
         1,
         1,
         1,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162,
-        162
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2,
+        2
       ]
     },
     {