[gldemo] add support for vertex absolute positioning

gltiles.vert

@@ -22,10 +22,13 @@ uniform LayerParams {
   // pixel size of each tile in this layer
   float tileSize;
 
-  // Implicit positioning system, where tiles are laid out as a dense grid
-  // indexed by gl_VertexID in row-major order starting at layer origin
-  // (defined by transform) and progressing in stride-siied rows of tiles.
+  // 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
+  // layer origin (defined by transform) and progressing in stride-sized rows.
   // Any offset attribute value is added to this value.
+  //
+  // Otherwise absolute positioning is used, relying on offset to position each
+  // tile (still relative to transform origin of course).
   int stride;
 };
 
@@ -54,10 +57,13 @@ void main(void) {
     return;
   }
 
-  vec2 loc = vec2(
-    float(gl_VertexID % stride),
-    float(gl_VertexID / stride)
-  );
+  vec2 loc;
+  if (stride > 0) {
+    loc = vec2(
+      float(gl_VertexID % stride),
+      float(gl_VertexID / stride)
+    );
+  }
   loc += 0.5;
   loc += offset;