DRI2: erase the offscreen framebuffer part on first buffer allocation

Do this mostly for security reasons. We don't want any application
to see whatever was last rendered by the previous GLES application
by just peeking into a freshly allocated DRI2 buffer.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

src/sunxi_mali_ump_dri2.c

@@ -360,6 +360,11 @@ static DRI2Buffer2Ptr MaliDRI2CreateBuffer(DrawablePtr  pDraw,
         window_state->pDraw = pDraw;
         HASH_ADD_PTR(private->HashWindowState, pDraw, window_state);
         DebugMsg("Allocate DRI2 bookkeeping for window %p\n", pDraw);
+        if (disp && can_use_overlay) {
+            /* erase the offscreen part of the framebuffer */
+            memset(disp->framebuffer_addr + disp->gfx_layer_size, 0,
+                   disp->framebuffer_size - disp->gfx_layer_size);
+        }
     }
     window_state->buf_request_cnt++;