Scaled CanvasLayer reports wrong area to child nodes

[cgbeutler]

Godot version:
3.2

OS/device including version:
Windows 10

Issue description:
When a CanvasLayer is scaled, it's children get the viewport size when requesting their parent's area size. This means that a scaled CanvasLayer's children will anchor wrong.

Steps to reproduce:

1. Create a 2DNode scene
2. Add a CanvasLayer with scale set to (2,2)
3. Add a Control as a child of the CanvasLayer and set it's Layout to Full Rect
   Note that the control is now twice the size of the actual viewport.

Minimal reproduction project:
https://github.com/cgbeutler/godot_bugs_CanvasLayerScaleAnchorsBug

[cgbeutler]

Seems like a CanvasLayers' area should be what is visible. In other words, it seems like the CanvasLayer should report it's area as Viewport / scale.
If the above is intended behavior, I will probably propose a feature to add a Scale Area var that can be set to the inverse of the scale option. Without it, anchors are kinda useless for children of CanvasLayers.

[cgbeutler]

A quick fix is to make a root control node inside the CanvasLayer and whenever the CanvasLayer scale is set, anchor the right and bottom of the root control to the inverse of the scale. Something like:

$CanvasLayer.scale = value
$CanvasLayer.RootControl.anchor_right = 1/value.x
$CanvasLayer.RootControl.anchor_bottom = 1/value.y

[cgbeutler]

A control calls the method scene/gui/Control.cpp > get_parent_anchorable_rect() to find it's anchor starting point. Since CanvasLayer is not a CanvasItem, it isn't actually used at all for these calculations. Instead, the function falls back to calling get_viewport()->get_visible_rect(). This gives you the viewport's visible rect without the canvas' transform applied at all. Not really sure the cleanest/simplest way to fix this.

[cgbeutler]

I figured out a minimal fix.
Basically, in get_parent_anchorable_rect(), the following line could be changed to use the global transform with canvas:

// Code was:
parent_rect = get_viewport()->get_visible_rect();
// Changed to:
parent_rect = get_global_transform_with_canvas().affine_inverse().xform(get_viewport()->get_visible_rect());

Not sure if there's a better way.

[cgbeutler]

Seems like the canvas layer transforms are left out all over the place. When opening a popup, the popup is clamped to a non-translated rect. There's also my other submitted bug: #35965