fix(IB): copy/paste error in the ImageBuf iterator copy constructor (#4365)

Fixes a simple copy/paste error in a copy constructor where the y
coordinate gets initialised twice instead of y and z.

Signed-off-by: Anton Dukhovnikov <[email protected]>

Author: antond-weta

src/libOpenImageIO/imagebuf.cpp

@@ -3167,7 +3167,7 @@ ImageBuf::IteratorBase::operator=(const IteratorBase& i)
     m_rng_zend   = i.m_rng_zend;
     m_x          = i.m_x;
     m_y          = i.m_y;
-    m_y          = i.m_y;
+    m_z          = i.m_z;
     return *this;
 }