Matrix is laid out in column-major order, so the stride between elements
on the main diagonal is #rows + 1, not #columns + 1.

Bug: b/116263076
Change-Id: I00994b663bc4229e24f8ae1b7fa518caa0e7ea71
Reviewed-on: https://swiftshader-review.googlesource.com/20768Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>