VLoc allows any location in the buffer to be represented as a visual
location in the linewrapped buffer. In particular, this is useful
for implementing moving cursor up and down within a wrapped line.
Fix displaying tabs and wide runes which don't fit in the window.
Don't overwrite the vertical divider and the adjacent window.
- For tabs: display only as many of the tab's spaces as fit in the window.
- For wide runes: if a rune doesn't fit, don't display it in this line at all.
If softwrap is on, display this rune in the next line.
Fixes#1979
Fixes issue with the usage of a slightly incorrect buffer height value
(v.Height should be v.Height-1 if statusline is displayed).
Also, to avoid too many duplications, the code reorganized a little:
buffer display params (width, height, gutter offset and others) are
calculated in a single place.
Softwrap implementation enhanced to fix various issues with scrolling,
centering, relocating etc.
The main idea is simple: work not with simple line numbers but
with (Line, Row) pairs, where Line is a line number in the buffer
and Row is a visual line (a row) number within this line.
The logic remains mostly the same, but simple arithmetic operations
on line numbers are replaced with corresponding operations on
(Line, Row) pairs.
Fixes#632, #1657
