Fix CheckBox and CheckButton not using icon_max_width

[sievaxx]

quick attempt. follows original rendering behavior in terms of where things are drawn if the other currently not drawn icons are larger.
fixes #91697

[sievaxx]

I don't think there's anything major i have to do, this seems like it's fine and everything else related to fitting the icons (e.g. different aspect ratios between all the state textures) seems to work the same as the old behavior (icons are drawn to the top left corner of the max size of all the icons), they're just now all able to be hard capped to a specific width.

I could add max_icon_width to the Checkbox and Checkbutton theme binds so they show up in theme overrides, but you can still add them manually to a theme resource and they inherit the settings of the button class, so dunno. This is only a minor usage (I was trying to make some UI look good at bigger render scales by scaling SVGs while importing and setting their icon width to the SVG's original size.).

[KoBeWi]

Might be worth documenting this in icon_max_width description. Something like "If the button has additional icons (e.g. [CheckBox]), they will also be limited."

[akien-mga]

Added KoBeWi's doc suggestion.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉