mesa: unreference current winsys buffers when unbinding winsys buffers

This fixes surface leak when no winsys buffers are bound.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/context.c

@@ -1765,6 +1765,10 @@ _mesa_make_current( struct gl_context *newCtx,
 
          check_init_viewport(newCtx, drawBuffer->Width, drawBuffer->Height);
       }
+      else {
+         _mesa_reference_framebuffer(&newCtx->WinSysDrawBuffer, NULL);
+         _mesa_reference_framebuffer(&newCtx->WinSysReadBuffer, NULL);
+      }
 
       if (newCtx->FirstTimeCurrent) {
          handle_first_current(newCtx);