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/command.lua

@@ -59,7 +59,7 @@ end
 
 
 function command.add_defaults()
-  local reg = { "core", "root", "command", "doc", "findreplace", "files" }
+  local reg = { "core", "root", "command", "doc", "findreplace", "files", "drawwhitespace" }
   for _, name in ipairs(reg) do
     require("core.commands." .. name)
   end

data/core/commands/drawwhitespace.lua

@@ -0,0 +1,16 @@
+local command = require "core.command"
+local config = require "core.config"
+
+command.add(nil, {
+  ["draw-whitespace:toggle"]  = function()
+    config.draw_whitespace = not config.draw_whitespace
+  end,
+
+  ["draw-whitespace:disable"] = function()
+    config.draw_whitespace = false
+  end,
+
+  ["draw-whitespace:enable"]  = function()
+    config.draw_whitespace = true
+  end,
+})

data/core/config.lua

@@ -21,6 +21,7 @@ config.max_project_files = 2000
 config.transitions = true
 config.animation_rate = 1.0
 config.blink_period = 0.8
+config.draw_whitespace = false
 
 -- Disable plugin loading setting to false the config entry
 -- of the same name.

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
 

data/core/init.lua

@@ -345,6 +345,14 @@ function core.remove_project_directory(path)
 end
 
 
+local function whitespace_replacements()
+  local r = renderer.replacements.new()
+  r:add(" ", "·")
+  r:add("\t", "»")
+  return r
+end
+
+
 function core.init()
   command = require "core.command"
   keymap = require "core.keymap"
@@ -416,6 +424,7 @@ function core.init()
   core.redraw = true
   core.visited_files = {}
   core.restart_request = false
+  core.replacements = whitespace_replacements()
 
   core.root_view = RootView()
   core.command_view = CommandView()