mesa: call FLUSH_VERTICES() before deleting shaders, buffers, query objects

Need to flush rendering (or at least indicate that the rug might be getting
pulled out from underneath us) when a shader, buffer object or query object
is about to be deleted.

Also, this helps to tell the VBO module to unmap its current vertex buffer.

src/mesa/main/bufferobj.c

@@ -749,6 +749,7 @@ _mesa_DeleteBuffersARB(GLsizei n, const GLuint *ids)
    GET_CURRENT_CONTEXT(ctx);
    GLsizei i;
    ASSERT_OUTSIDE_BEGIN_END(ctx);
+   FLUSH_VERTICES(ctx, 0);
 
    if (n < 0) {
       _mesa_error(ctx, GL_INVALID_VALUE, "glDeleteBuffersARB(n)");