Add early campaign battlefield backgrounds

data/scenarios/002_sishui_gate.json

@@ -46,6 +46,7 @@
   "map": {
     "width": 12,
     "height": 9,
+    "background": "res://art/backgrounds/sishui_gate_pass.png",
     "terrain": [
       "GGGGGRRGGGGG",
       "GGGFFRRGHHGG",

data/scenarios/008_wan_castle_escape.json

@@ -74,6 +74,7 @@
   "map": {
     "width": 15,
     "height": 10,
+    "background": "res://art/backgrounds/wan_castle_escape.png",
     "terrain": [
       "GGGGGHHGGGFFFFG",
       "GGFFGHHGGGFFFFG",

data/scenarios/009_xiapi_siege.json

@@ -67,6 +67,7 @@
   "map": {
     "width": 15,
     "height": 10,
+    "background": "res://art/backgrounds/xiapi_flooded_siege.png",
     "terrain": [
       "GGGGGHHGGGWWGGG",
       "GGGFFHHGGGWWGGG",

data/scenarios/012_guandu_showdown.json

@@ -67,6 +67,7 @@
   "map": {
     "width": 17,
     "height": 11,
+    "background": "res://art/backgrounds/guandu_defensive_camps.png",
     "terrain": [
       "GGGGGHHGGGGHHGGGG",
       "GGFFGHHGGFFHHGFFG",