perform convolution properly independently of matrix orientation (bug #34893) * cconv2.f (cconv2i), csconv2.f (csconv2i), dconv2.f (dconv2i), sconv2.f (sconv2i), zconv2.f (zconv2i), zdconv2.f (zdconv2i): Fix iteration over the a matrix, so it's properly reversed. * conv2.cc: Uncomment test to check for this, and add a new xtest.

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