[gldemo] switch to gl_VertexIndex

gltiles.vert

@@ -23,7 +23,7 @@ uniform LayerParams {
   float tileSize;
 
   // Implicit positioning mode is enabled if stride > 0, where tiles are laid
-  // out as a dense grid indexed by gl_VertexID in row-major order starting at
+  // out as a dense grid indexed by gl_VertexIndex in row-major order starting at
   // layer origin (defined by transform) and progressing in stride-sized rows.
   // Any offset attribute value is added to this value.
   //
@@ -60,8 +60,8 @@ void main(void) {
   vec2 loc;
   if (stride > 0) {
     loc = vec2(
-      float(gl_VertexID % stride),
-      float(gl_VertexID / stride)
+      float(gl_VertexIndex % stride),
+      float(gl_VertexIndex / stride)
     );
   }
   loc += 0.5;