Add some FLUSH_CURRENT() calls.

Without these we can find ourselves in _mesa_load_state_parameters()
computing derived lighting/material values whhen the current material
properties haven't been updated from the VBO.
This may be a somewhat wide-spread problem that needs more attention...
(cherry picked from commit 49adf51eee)

src/mesa/main/buffers.c

@@ -118,6 +118,8 @@ _mesa_Clear( GLbitfield mask )
    GET_CURRENT_CONTEXT(ctx);
    ASSERT_OUTSIDE_BEGIN_END_AND_FLUSH(ctx);
 
+   FLUSH_CURRENT(ctx, 0);
+
    if (MESA_VERBOSE & VERBOSE_API)
       _mesa_debug(ctx, "glClear 0x%x\n", mask);