fix copyteximage piglit failures
For texture arrays, the layer is stored at blit.{src,dst}.box.z. In those cases, we need to go through the same path as we already do for GL_TEXTURE_CUBE_MAP.
Fixes piglit's spec@ext_texture_array@copyteximage {1d_array,2d_array}
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Tested on both GLES and GL backends.
Edited by Tomeu Vizoso