Use the new gegl_tile_read_lock() and gegl_tile_read_unlock()
functions, added in the previous commit, before/after reading from
a tile in a potentially-multithreaded context.

See the previous commit for an explanation of why this is
necessary.

Note, in particular, that this may make 1x1 gegl_buffer_get()s
(and NEAREST gegl_buffer_sample()) more expensive, however, NEAREST
sampler objects amortize the cost of these functions by only
locking/unlocking the sampler's hot-tile when switching tiles.  As
a result, a concurrent write to the same buffer may block while the
sampler object is alive.  We assume that buffers may not be
written-to while there are active sampler objects constructed for
them (which can already lead to stale data in the samplers).