Fix libc++abi.dylib: Pure virtual function called!

On MacOS, we may end up with the following runtime error.
OpenEXR/dwaLookups.h
libc++abi.dylib: Pure virtual function called!

Fix dwaLookup generation which is due to calling the base function from constructor which calls the overriden method run, which causes the base class to call the pure virtual method only, which does not do anything. outfile ended up not being generated.

OpenEXR/IlmImf/dwaLookups.cpp

@@ -75,7 +75,6 @@ namespace {
                         _worker(worker),
                         _output(output)
                     {
-                        start();
                     }
 
                     virtual ~Runner()
@@ -512,6 +511,7 @@ generateLutHeader()
         std::vector<LutHeaderWorker::Runner*> runners;
         for (size_t i=0; i<workers.size(); ++i) {
             runners.push_back( new LutHeaderWorker::Runner(*workers[i], (i==0)) );
+            runners.back()->start();
         }
 
         for (size_t i=0; i<workers.size(); ++i) {