dri: Store the loader private passed in at dri context creation

We just threw it away before, but we haven't had a use for it yet.

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

@@ -581,7 +581,8 @@ driCreateNewContext(__DRIscreen *psp, const __DRIconfig *config,
 
     pcp->driScreenPriv = psp;
     pcp->driDrawablePriv = NULL;
-
+    pcp->loaderPrivate = data;
+    
     /* When the first context is created for a screen, initialize a "dummy"
      * context.
      */

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

@@ -413,6 +413,11 @@ struct __DRIcontextRec {
      * Pointer to screen on which this context was created.
      */
     __DRIscreen *driScreenPriv;
+
+    /**
+     * The loaders's private context data.  This structure is opaque.
+     */
+    void *loaderPrivate;
 };
 
 /**