Added more asserts for dist-8x8 running in PSNR mode,
i.e. with the encoder option "--enable-dist-8x8=1"
instead of --tune=[cdef-dist | daala-dist].

The asserts checks whether a 8x8 distortion measured on reconstructed 8x8 pixels
is identical to the sum of distortions from sub8x8 partitions (or tx blocks for
var-tx case).

Change-Id: I14f2b24e674a9cbbe60e663449fc4e7f46f0e481