renderer fonts: additions and improvements

* Allow passing font options to renderer.font:copy().
* Added renderer.font:get_path()
* Reintroduced set_size() for more faster font size changes
* Swapped copy wiht set_size on scale plugin for better performance
* Use code_font:copy() instead of renderer.font.load() on language_md to
  properly match user font now that font options are supported on copy.
* Added new changes to renderer docs

data/plugins/language_md.lua

@@ -14,9 +14,7 @@ for _, attr in pairs({"bold", "italic", "bold_italic"}) do
     attributes["bold"] = true
     attributes["italic"] = true
   end
-  -- no way to copy user custom font with additional attributes :(
-  style.syntax_fonts["markdown_"..attr] = renderer.font.load(
-    DATADIR .. "/fonts/JetBrainsMono-Regular.ttf",
+  style.syntax_fonts["markdown_"..attr] = style.code_font:copy(
     style.code_font:get_size(),
     attributes
   )