gallium: new pipe->texture_update() function

Called whenever texture data is changed (glTexImage, glTexSubImage,
glCopyTexSubImage, etc).

src/gallium/include/pipe/p_context.h

@@ -206,6 +206,14 @@ struct pipe_context {
    void (*texture_release)(struct pipe_context *pipe,
 			   struct pipe_texture **pt);
 
+   /**
+    * Called when texture data is changed.
+    * Note: we could pass some hints about which mip levels or cube faces
+    * have changed...
+    */
+   void (*texture_update)(struct pipe_context *pipe,
+                          struct pipe_texture *texture);
+
    /** Get a surface which is a "view" into a texture */
    struct pipe_surface *(*get_tex_surface)(struct pipe_context *pipe,
                                            struct pipe_texture *texture,