Add portable Windows build and update GC runtime

docs/stage.pat

@@ -186,6 +186,21 @@ struct VisualizerArray {
     Visualizer entries[sz];
 };
 
+// Background Images
+struct BackgroundImage {
+    u32 timeMs;
+    u32 mode;
+    u32 atlasIndex;
+    s32 textureIndex;
+    color color;
+} [[same_color]];
+struct BackgroundImageArray {
+    u32 textureCount;
+    string8 textureNames[textureCount];
+    u32 sz;
+    BackgroundImage entries[sz];
+};
+
 // Color Table 1
 struct ColorTable {
     u32 timeMs;
@@ -327,6 +342,8 @@ NoteArray notes @ hdr.notes;
 CameraArray camera @ hdr.camera;
 ParticleArray particles @ hdr.particles;
 VisualizerArray visualizer @ hdr.visualizer;
+BackgroundImageArray backgroundImages @ hdr.unk1;
 ColorTableArray colorTable @ hdr.colors;
 ObjectArray objects @ hdr.objects;
-ColorTable2Array colorTable2 @ hdr.colors2;
+// Header word 11 points to offset slot 2, one word into this table.
+ColorTable2Array colorTable2 @ hdr.colors2 - 4;