fix position translation at EOF with softwrap

[pascalkuthe]

Fixes #5782

The new rendering code added in #5420 performs char_idx -> visual_pos translations on the fly for performance reasons. This code needs a special case for the EOF character as that character is not a real char_idx in the text (it's rendered as virtual text but virtual text normally doesn't have a separate char_idx). When softwrap is enabled the view may be offset multiple visual lines from the start of the line. That means that any position needs to have a vertical offset applied (pos.row -= row_offset). This is always done for all graphemes that get actually rendered but because the EOF position takes a specical branch (only used for position translation) it needs to be done there too (which was missed so far).