Given matrix M_in, flip every pair of columns. So if M_in is

 1 2 3 4
 1 2 3 4

then M_out is

 2 1 4 3
 2 1 4 3

Note: if M_in has odd number of columns, the last column should remain unchanged. So if M_in is

 17    24     1     8    15
 23     5     7    14    16
  4     6    13    20    22
 10    12    19    21     3
 11    18    25     2     9

then M_out is

 24    17     8     1    15
  5    23    14     7    16
  6     4    20    13    22
 12    10    21    19     3
 18    11     2    25     9