Mostly fix problem of offset with cursor positioning when using mouse

In order to get right the cursor position on text on mouse clicks it is
needed to take into account text's subpixel positioning.

This fix mostly corrects the problem but cursor positioning is still
somewhat inaccurate for long lines due to repeated commands to draw a text
along a line. Repeated draw text calls make the subpixel information
lost and small errors will add-up.

data/core/docview.lua

@@ -152,10 +152,12 @@ function DocView:get_x_offset_col(line, x)
   local text = self.doc.lines[line]
 
   local xoffset, last_i, i = 0, 1, 1
+  local subpixel_scale = self:get_font():subpixel_scale();
+  local x_subpixel = subpixel_scale * x + subpixel_scale / 2
   for char in common.utf8_chars(text) do
-    local w = self:get_font():get_width(char)
-    if xoffset >= x then
-      return (xoffset - x > w / 2) and last_i or i
+    local w = self:get_font():get_width_subpixel(char)
+    if xoffset >= subpixel_scale * x then
+      return (xoffset - x_subpixel > w / 2) and last_i or i
     end
     xoffset = xoffset + w
     last_i = i