fix some warnings, disable gimpact by default in world importer,

use DIRECTLY_UPDATE_VELOCITY_DURING_SOLVER_ITERATIONS by default for now,
until we find the issue with some failing test cases in btMultiBody
fix a crashing issue in MyMultiBodyCreator.cpp (uninitialized variable)
disable excessive debug printf in URDF2Bullet

examples/ThirdPartyLibs/Gwen/Gwen.h

@@ -24,7 +24,6 @@
 	THE SOFTWARE.
 */
 
-#pragma once
 #ifndef GWEN_GWEN_H
 #define GWEN_GWEN_H
 

examples/ThirdPartyLibs/Wavefront/tiny_obj_loader.cpp

@@ -171,7 +171,7 @@ updateVertex(
     return it->second;
   }
 
-  assert(in_positions.size() > (3*i.v_idx+2));
+  assert(static_cast<int>(in_positions.size()) > (3*i.v_idx+2));
 
   positions.push_back(in_positions[3*i.v_idx+0]);
   positions.push_back(in_positions[3*i.v_idx+1]);