show trails while moving cursor, within a window or across windows
- works when cursor moving within one single window
- works when cursor moving across windows, yet there would be break points
- since nvim_buf_set_extmark doesnt supports per-window-based mark, fn.matchadd* must be used
- no fullscreen floatwin + winblend, because i dont use &termguicolor
- no massive ephemeral floatwins, no pre-alloc floatwins
- should be ok with multi-bytes strings
- not supposed to work well with , which may be 2/4/8-width
- just works, imperfectly
- yet many untested edge cases: signcolumn, numbercolumn, tabline, window-statusline, window-border, winbar ...
- since it requires a patched vim.fn.getmousepos, it's not supposed to be used publicly
- i shamelessly stole the basis impl from vim-ranbow-trails
- the bresenham algo