fixing error undef ref for win32

core/image/GrayColorImageCommonImplementation.cpp

@@ -10,6 +10,9 @@
 #include <sstream>
 
 namespace NICE {
+//Note: You must define the static instance, not just declare it. The definition creates the actual object you refer to. Without this line
+//Visual Studio returns an undefined reference error
+const GrayColorImageCommonImplementation::ShallowCopyMode GrayColorImageCommonImplementation::shallowCopy;
 
 GrayColorImageCommonImplementation::GrayColorImageCommonImplementation() {
   m_memoryLayout = internal__foreignPointer;