fixup! [gldemo] allow tile position data to be animated

gltiles.vert

@@ -43,6 +43,8 @@ uniform LayerParams {
 in uint layerID;
 
 in vec4 pos;
+in vec4 pos_to;
+in vec3 anim; // x=start_time, y=duration, z=mode
 
 out float sheetLayer;
 out mat3 tileTransform;
@@ -54,6 +56,32 @@ const mat3 spinOffset = mat3(
 );
 
 void main(void) {
+  vec4 pos_at = pos;
+
+  if (anim_enabled && anim.y > 0.0) {
+    float a = (anim_time - anim.x) / anim.y;
+    if (a > 0.0) {
+      switch (int(anim.z)) {
+
+        case 1: // loop
+          a = fract(a);
+          break;
+
+        case 2: // loop-back
+          if (int(floor(a)) % 2 == 1) {
+            a = 1.0 - fract(a);
+          } else {
+            a = fract(a);
+          }
+          break;
+
+        default:
+          a = max(0.0, min(1.0, a));
+      }
+      pos_at = mix(pos_at, pos_to, /*a*/ 0.0);
+    }
+  }
+
   vec2 offset = pos.xy;
   float spin = pos.z;
   float scale = pos.w;