Windows: fewer windows.h includes from platform.h (#3596)

In particular, defining NOGDI before including <windows.h> achieves
several things amongh others:
- No longer causes ERROR symbol to be defined to 0. ERROR being defined
  prevents using e.g. google/glog library, since it wants ERROR to mean
  something else.
- No longer defines some possibly widely used names e.g. GetObject.

Author: aras-p

src/include/OpenImageIO/platform.h

@@ -48,6 +48,9 @@
 #    ifndef NOMINMAX
 #        define NOMINMAX
 #    endif
+#    ifndef NOGDI
+#        define NOGDI
+#    endif
 #    include <windows.h>
 #endif