opengl32: Make it possible to use an alternative in wglGetProcAddress when an extension is missing.

dlls/opengl32/wgl.c

@@ -878,6 +878,19 @@ PROC WINAPI wglGetProcAddress( LPCSTR name )
 
         if (!is_extension_supported(ext_ret->extension))
         {
+            unsigned int i;
+            static const struct { const char *name, *alt; } alternatives[] =
+            {
+                { "glCopyTexSubImage3DEXT", "glCopyTexSubImage3D" },  /* needed by RuneScape */
+            };
+
+            for (i = 0; i < sizeof(alternatives)/sizeof(alternatives[0]); i++)
+            {
+                if (strcmp( name, alternatives[i].name )) continue;
+                WARN("Extension %s required for %s not supported, trying %s\n",
+                    ext_ret->extension, name, alternatives[i].alt );
+                return wglGetProcAddress( alternatives[i].alt );
+            }
             WARN("Extension %s required for %s not supported\n", ext_ret->extension, name);
             return NULL;
         }