Fix SubViewport not rendering correctly

[Rindbee]

Fix #55471.

Previously, the necessary steps in _set_size were not performed when the SubViewport was initialized, which caused rendering issues. There was no problem when changing the parameters (make size, size_2d_override, or size_2d_override_stretch not the default value) which caused _set_size to be called.

[Rindbee]

When the SubViewport enters the tree, some properties should be set according to its parent.

The parent SubViewportContainer should also be updated when the SubViewport changes as follows:
1. Size changed (or update);
2. Enter/exit the tree.

Changes since last review

[Rindbee]

godot/scene/main/viewport.h

Lines 727 to 736 in 836fe9a

	enum UpdateMode {
	UPDATE_DISABLED,
	UPDATE_ONCE, //then goes to disabled
	UPDATE_WHEN_VISIBLE, // default
	UPDATE_WHEN_PARENT_VISIBLE,
	UPDATE_ALWAYS
	};
	private:
	UpdateMode update_mode = UPDATE_WHEN_VISIBLE;

Currently the default update_mode is UPDATE_WHEN_VISIBLE, which doesn't make sense because SubViewport has no concept of visibility. We might need to choose a meaningful update_mode.

Of course, update_mode (including clear_mode) will not take effect, because it is not used in RenderingServer when initialized.

Given the close relationship between SubViewport and SubViewportContainer, the interaction between them is considered comprehensively.

[Rindbee]

UPDATE_ALWAYS might be better, but I'm not sure if it's necessary.

[Rindbee]

Split into #64138 and #64152.