Introduce subpixel text positioning within rencache

In order to fix the issue with cursor positioning a subpixel-aware draw
text operation within rencache was required.

With this modification the cursor positioning problem is completely
resolved.

A new function renderer.draw_text_subpixel is introduced to perform
consecutive, inline, text drawings with subpixel accuracy.

data/core/docview.lua

@@ -309,11 +309,12 @@ end
 
 
 function DocView:draw_line_text(idx, x, y)
-  local tx, ty = x, y + self:get_line_text_y_offset()
   local font = self:get_font()
+  local subpixel_scale = font:subpixel_scale()
+  local tx, ty = subpixel_scale * x, y + self:get_line_text_y_offset()
   for _, type, text in self.doc.highlighter:each_token(idx) do
     local color = style.syntax[type]
-    tx = renderer.draw_text(font, text, tx, ty, color)
+    tx = renderer.draw_text_subpixel(font, text, tx, ty, color)
   end
 end