big architecture change dissociating "actual" dimensions from "maximum possible"

dimension. The advantage is that evaluating a dynamic-sized block in a fixed-size
matrix no longer causes a dynamic memory allocation. Other new thing:
IntAtRunTimeIfDynamic allows storing an integer at zero cost if it is known at
compile time.

Eigen/src/Core/MatrixRef.h

@@ -41,7 +41,9 @@ template<typename MatrixType> class MatrixRef
   private:
     enum {
       RowsAtCompileTime = MatrixType::Traits::RowsAtCompileTime,
-      ColsAtCompileTime = MatrixType::Traits::ColsAtCompileTime
+      ColsAtCompileTime = MatrixType::Traits::ColsAtCompileTime,
+      MaxRowsAtCompileTime = MatrixType::Traits::MaxRowsAtCompileTime,
+      MaxColsAtCompileTime = MatrixType::Traits::MaxColsAtCompileTime
     };
 
     int _rows() const { return m_matrix.rows(); }