Add IAT based hooking to allow hooking 64bit opengl32.dll

* Needs vigorous testing in as many situations as I can think of, not least
  32bit, 64bit, D3D11, D3D9, OpenGL, managed apps, different windows SDKs,
  delay loading dlls (not sure if these will work), statically linked libs,
  LoadLibrary + GetProcAddress
* Credit goes to jslutter for originally bringing it to my attention &
  suggesting it.
* References:
  http://msdn.microsoft.com/en-us/magazine/bb985992.aspx
  http://msdn.microsoft.com/en-us/magazine/cc301808.aspx
  http://www.codeproject.com/Articles/6265/Process-wide-API-spying-an-ultimate-hack

renderdoc/hooks/gl_win32_hooks.cpp

@@ -228,11 +228,13 @@ class OpenGLHook : LibraryHook
 			if(!m_EnabledHooks)
 				return false;
 			
+#if USE_MHOOK
 			if(GetModuleHandleA(DLL_NAME) == NULL)
 			{
 				RDCWARN("Failed to load %s - not inserting OpenGL hooks.", DLL_NAME);
 				return false;
 			}
+#endif
 
 			bool success = SetupHooks(GL);