Win32: Allow compile-time override of class name

To use, define _GLFW_WNDCLASSNAME to any sane wide string.

Fixes #1120.

src/win32_platform.h

@@ -242,7 +242,9 @@ typedef VkBool32 (APIENTRY *PFN_vkGetPhysicalDeviceWin32PresentationSupportKHR)(
 #include "egl_context.h"
 #include "osmesa_context.h"
 
-#define _GLFW_WNDCLASSNAME L"GLFW30"
+#if !defined(_GLFW_WNDCLASSNAME)
+ #define _GLFW_WNDCLASSNAME L"GLFW30"
+#endif
 
 #define _glfw_dlopen(name) LoadLibraryA(name)
 #define _glfw_dlclose(handle) FreeLibrary((HMODULE) handle)