ARVR viewport size cannot be changed

[mittorn]

Godot version

3.x branch

System information

any

Issue description

It is impossible to override arvr viewport size. When rendering overlay openvr application which has multiple viewports, all ARVR viewpors are forced to main rendertarget size: https://github.com/godotengine/godot/blob/3.x/servers/visual/visual_server_viewport.cpp#L288 This results in non pixel-perfect overlay rendering and high gpu usage even on small overlays It cannot be solved on godot_openvr side because godot_openvr does not know which rendertarget size is requested with ARVR interface, it only can report same value for all overlays as it sorted every frame. Adding some viewport info or some flag that disables this lines would solve this

Steps to reproduce

Create viewport, enable arvr flag Set viewport size get it reseted every frame before rendering witn no ability to somehow hook/override it even with custom ARVR interface

Minimal reproduction project

N/A

[Calinou]

• Related to https://github.com/godotengine/godot/pull/74935.

cc @BastiaanOlij

[mittorn]

PR not related to vr viewports as it's size is overriden