app: invalidate channel boundary upon buffer "changed" signal
Have GimpChannel connect to the drawable buffer's "changed" signal, so that we can invalidate the channel's boundary whenever the buffer contents change. Currently, the calls to gimp_drawable_invalidate_boundary() dispersed throughout the code are not enough. Moreover, invalidate both the boundary and the bounds in gimp_channel_invalidate_boundary(), since both are necessary when the buffer changes.
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