Enable OpenVR flat screen submissions

[mutars]

Motivation

• Allow the flat-screen/quad display setting to be used with OpenVR instead of being blocked by a hardcoded HMD check.
• When flat-screen mode is selected, present the same rendered image to both OpenVR eye layers so desktop/flat output can be recorded or shown correctly.

Description

• Removed the OpenVR-specific early-return in ModSettings::showFlatScreenDisplay() so the flat screen setting can be used for OpenVR (updated src/ModSettings.cpp).
• Added a flat_openvr check and flat-screen submission path to the D3D11 renderer that duplicates the eye texture into the opposite-eye texture and submits both eyes (updated src/mods/vr/D3D11Component.cpp).
• Added ModSettings.h include and the same flat-screen submission logic to the D3D12 renderer, duplicating the eye texture for both eyes and submitting both layers when enabled (updated src/mods/vr/D3D12Component.cpp).
• Adjusted the pose/bounds handling in the flat mode branches so both left and right eye submits use the duplicated textures and are submitted to the compositor.

Testing

• No automated tests were executed for this change.

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Codex Task