In GimpProjection, respond to the projectable's "bounds-changed"
signal, by reallocating the buffer, and copying the corresponding
region of the old buffer (using
gimp_tile_handler_validate_buffer_copy(), added a few commits back,
so that the relevant portion of the validate handler's dirty region
is also copied).  Additionally, shift and clip all outstanding
update regions as necessary (actually, we avoid copying the buffer
when a shift is necessary, and simply reconstruct the projection;
see FIXME comment in the code.)

(cherry picked from commit fbeae361)