Fix Viewport size change not updating textures

[KoBeWi]

Fixes this bug:

Test script (3.x):

func _ready() -> void:
	yield(get_tree().create_timer(1), "timeout")
	$Viewport.size *= 2
	yield(get_tree().create_timer(1), "timeout")
	$PanelContainer/Sprite.update()

After fix:

Originally discovered in 3.x, but the bug also exists on master.

Here's code for 3.x branch:

for (Set<ViewportTexture *>::Element *E = viewport_textures.front(); E; E = E->next()) {
	E->get()->emit_changed();
}

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.6.