Math Eval for rectangular selection

[ProgerXP]

Allow evaluating math for rectangular selection (Alt+drag), for example, when a column is selected in this text (result=15):

[cshnik]

Done.

[ProgerXP]

Invalid result if rectangular selection contains blank lines:

1 [2]

3 [4]

Select 3rd column with Alt - result will be either 2 or 4 depending on if you select top down or the other way around.

[cshnik]

Fixed.