Perfect FreeType-based outlines for DynamicFonts

- Implement outlines based on FreeType Stroker API. This allows
  artifact-free results, similar to what you will see in Web or any text
  editing tools. Outline is a part of DynamicFont rather than Label,
  because outlines have to be baked into the font's atlas. Font has a
  default outline_color and a Label can specify font_outline_modulator
  that will be multiplied with the Font's color to get the final result.

- draw_char now has to be called twice to fully render a text - first
  with p_outline == true for each character and then with
  p_outline == false for each character.

- Number of draw-calls is reduced from 5 to 2 per outlined character.

- Overall cleanup of DynamicFont code, extracted duplicated code pieces
  into separate methods.

- The change is backward-compatible - Labels still have outline
  properties that work exactly as they worked before.

Closes #16279.

scene/resources/default_theme/default_theme.cpp

@@ -415,6 +415,7 @@ void fill_default_theme(Ref<Theme> &theme, const Ref<Font> &default_font, const
 
 	theme->set_color("font_color", "Label", Color(1, 1, 1));
 	theme->set_color("font_color_shadow", "Label", Color(0, 0, 0, 0));
+	theme->set_color("font_outline_modulator", "Label", Color(1, 1, 1));
 
 	theme->set_constant("shadow_offset_x", "Label", 1 * scale);
 	theme->set_constant("shadow_offset_y", "Label", 1 * scale);