In gegl_buffer_copy() and gegl_buffer_clear(), calculate tile-grid-
aligned cow_rect directly instead of using loops, avoid splitting
non-COWed area when cow_rect is empty, and some cleanup.

Fix a bug in gegl_buffer_clear(), where clearing a region that
doesn't cross tile boundary in either direction, erroneously
cleared the entire area from the top/left edge of the region to
the right/bottom edge of the containing tile, in the respective
direction.

Add a test for the above bug, for all of gegl_buffer_{clear,
set_color,set_pattern,copy}().