Add `Debug2D` singleton

[Xrayez]

See discussion #161, this is meant to resolve godotengine/godot-proposals#112, resolve godotengine/godot-proposals#5196.

Notable features include:

• Draw outside _draw() calls.
• Make snapshots of draw commands and play them back (useful for visualization).
• Inject draw calls to custom objects (can still override _draw() for each node if needed).
• Call custom draw methods defined via script.

To-do

• [x] Implement useful draw methods.
• [x] Rewrite or adapt the drawing API to remove bloat for parameters which are used often like color, line width, filled etc (via draw_set_color(), draw_set_filled()).
• [ ] Add tags in order to filter captured draw commands (?)
• [x] Add project settings for configuring default draw parameters.

Example

extends Node2D

func _ready():
    Debug2D.draw_circle(32, Vector2(), Color.orange)
    Debug2D.capture()

    Debug2D.canvas_item = $RigidBody2D
    Debug2D.draw_circle(32, Vector2(), Color.orange)
    Debug2D.capture()

    Debug2D.canvas_item = Debug2D.get_base()
    Debug2D.draw("rect", [Rect2(0, 0, 100, 100), Color.steelblue])
    Debug2D.draw_circle(32, Vector2(), Color.orange)
    Debug2D.capture()

func _input(event):
    if event.is_action_pressed("ui_left"):
        Debug2D.get_capture().draw_prev()
    elif event.is_action_pressed("ui_right"):
        Debug2D.get_capture().draw_next()

    if event.is_action_pressed("ui_cancel"):
        Debug2D.get_capture().reset()

Test project debug_2d.zip

[Xrayez]

Implemented draw_arrow() now.

Pushed a snippet in GDScript which I used during prototyping, in case modifications are needed: https://github.com/goostengine/goost-plugins/commit/354d4a07a076fcb371ce6e555abb6f8991bd32d4

[Xrayez]

I'm more or less satisfied with current API. Further enhancements and changes are fine to make further down the road because it only affects debug builds.

Things to do:

• [ ] Add fixed canvas layer for drawing UI in general
• [ ] Add grid similar to goostengine/goost-plugins#1
• [ ] ... and more!