Fix Ogre2RenderTarget::TargetFSAA method

The Ogre2RenderTarget::TargetFSAA method has a logic to check if the selected antialiasing level is supported.
If the requested antialiasing level is not supported, the 0 level is selected instead. Furthermore, a warning is 
printed only once for process. Before this PR, the 0 level was only set only when the warning was printed, 
resulting in an unsupported antialiasing being set if Ogre2RenderTarget::TargetFSAA was called two or more times.

ogre2/src/Ogre2RenderTarget.cc

@@ -575,9 +575,9 @@ uint8_t Ogre2RenderTarget::TargetFSAA() const
       ignwarn << "Anti-aliasing level of '" << this->antiAliasing << "' "
               << "is not supported; valid FSAA levels are: " << os.str()
               << ". Setting to 0" << std::endl;
-      targetFSAA = 0u;
       ogre2FSAAWarn = true;
     }
+    targetFSAA = 0u;
   }
 
   if (targetFSAA == 0u)