Implement unicode character replacements

Useful to draw whitespaces with alternate characters and colors
without slowing down the text rendering.

A new API is implemented. A renderer.replacements object can be created
to list the replacements.

In turns the function renderer.draw_text and draw_text_subpixel now accept
two optional arguments for replacements.

data/core/docview.lua

@@ -314,7 +314,11 @@ function DocView:draw_line_text(idx, x, y)
   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_subpixel(font, text, tx, ty, color)
+    if config.draw_whitespace then
+      tx = renderer.draw_text_subpixel(font, text, tx, ty, color, core.replacements, style.syntax.comment)
+    else
+      tx = renderer.draw_text_subpixel(font, text, tx, ty, color)
+    end
   end
 end