fix virtual col for multibyte JK motion when g:EasyMotion_startofline=0

[SolaWing]

Currently, when do jk motion at multibyte line, the col is incorrect. like following img

the virtcol('.') is also slightly incorrect for returning the last column when char occupying multi column.

after searching in vim's function, I found getcurpos returns curswant, which is the perfect choice .

The "curswant" number is the preferred column when moving the
cursor vertically.