Improve page specificity

Signed-off-by: Justin Georgi <justin.georgi@gmail.com>

includes/GlModelViewer.php

@@ -16,7 +16,8 @@ class GlModelViewer extends ImageHandler {
   }
 
   public static function onBeforePageDisplay(OutputPage $out) {
-    if (strpos($out->getHTML(), 'class="glmv-viewer') !== false) {
+    preg_match('/(<a href="\S*?\.(glb|gltf"))/',$out->getHTML(),$findGltf);
+    if ($findGltf[0]) {
       $out->addModules('ext.glmv');
     }
   }