[bug] DocView:get_line_screen_position() returns inacurate x-coordinate

[SwissalpS]

to reproduce:

• clone/checkout master at [38bd9b3] (latest as of this writing)
• build it
• add a plugin like this:

local DocView = require "core.docview"

local omr = DocView.on_mouse_released
function DocView:on_mouse_released(b, x, y)
  local line, col = self:resolve_screen_position(x, y)
  local x2, y2 = self:get_line_screen_position(line, col)
  print('x: ' .. x .. ' x2: ' .. x2)
  return true
end

• launch lite from terminal
• open a document
• click in the middle of a line and read output
  x ~= x2
  a small diff can be expected from rounding and a bigger one
  can be expected when clicking out to the right of text.
  In my tests the diff was over 100 pixels.