compile with -Wall, fix a warning

SConstruct

@@ -1,6 +1,6 @@
 
 # vim:filetype=python
 
-env = Environment(LIBS = ['GL'])
+env = Environment(LIBS = ['GL'], CXXFLAGS = ['-Wall'])
 
 env.Program('driver', Glob('*.cc'))

WFObj.cc

@@ -122,7 +122,7 @@ WFObj::~WFObj()
 
 void WFObj::clear()
 {
-    for (int i = 0; i < sizeof(m_vertices)/sizeof(m_vertices[0]); i++)
+    for (size_t i = 0; i < sizeof(m_vertices)/sizeof(m_vertices[0]); i++)
         m_vertices[i].clear();
     m_faces.clear();
     m_materials.clear();