Add a DRI_ReadDrawable marker extension to signal read drawable capability.

src/mesa/drivers/dri/common/dri_util.c

@@ -46,6 +46,14 @@ const __DRIinterfaceMethods * dri_interface = NULL;
 static const int empty_attribute_list[1] = { None };
 
 
+/**
+ * This is just a token extension used to signal that the driver
+ * supports setting a read drawable.
+ */
+const __DRIextension driReadDrawableExtension = {
+    __DRI_READ_DRAWABLE
+};
+
 /**
  * Cached copy of the internal API version used by libGL and the client-side
  * DRI driver.