FIRE-35062/SL-19968 & possibly #4014 - vanishing meshes after TP/login

There may be other bugs with similar symptoms, however.
Having spent a lot of time tracking specific attachments through teleports and examining the absence or not, this fix directly addresses a common situation whereby post-TP calls to rebuildGeom can query the region for "MeshRez" support before the caps have been received resulting in the mesh being eliminated until the next rebuildGeom()

indra/newview/llviewerregion.cpp

@@ -3985,6 +3985,13 @@ bool LLViewerRegion::bakesOnMeshEnabled() const
 
 bool LLViewerRegion::meshRezEnabled() const
 {
+    // <FS:Beq> FIRE-35602 and many similar reports - Mesh not appearing after TP/login
+    if(!mSimulatorFeaturesReceived)
+    {
+        LL_INFOS("MeshRez") << "MeshRezEnabled: SimulatorFeatures not received yet. Defaulting to true" << LL_ENDL;
+        return true;
+    }
+    // </FS:Beq>
     return (mSimulatorFeatures.has("MeshRezEnabled") &&
                 mSimulatorFeatures["MeshRezEnabled"].asBoolean());
 }