Ensure comma initializer reuses fixed dimensions

Reference issue

This relates to #2346 (closed).

What does this implement/fix?

Ensures that the block in CommaInitializer is fix-sized if the input is by replacing

m_xpr.block(0, 0, other.rows(), other.cols()) = other;

with

m_xpr.template block<OtherDerived::RowsAtCompileTime, OtherDerived::ColsAtCompileTime>(0, 0, other.rows(), other.cols()) = other;

Additional information

No additional information :)