[WIIU] Overlays are always full_screen

Description

Some overlays are intended to only fill the game area, like several of those in https://github.com/libretro/common-overlays/tree/master/effects/scanlines

These overlays behave the same as overlays with overlay0_full_screen = true, filling the entire screen with scanlines rather than just the game image.

Expected behavior

The default setting for overlay0_full_screen is false, so any overlay that either doesn't include this setting or sets it to false should only fill the game area.

Actual behavior

They fill the entire screen.

Steps to reproduce the bug

1. Set a directory for overlays in the directory settings (Wii U has no default overlay directory).
2. Download the overlays.
3. Enable any overlay that is intended to display only over the game image.
4. Observe that the overlay fills the entire screen.

Bisect Results

I didn't bisect, I don't think anybody really uses overlays on Wii U so it's pretty likely it's just always been this way and nobody noticed.

Version/Commit

• RetroArch: 4ffe9dc

Environment information

• OS: Wii U, 5.5.5 PAL
• Compiler: Official libretro-build-wiiu Docker container

Start by tracing Wii U overlay rendering and the handling of the overlay0_full_screen setting, comparing overlays that omit it or set it to false with those that set it to true. Reproduce the issue with a scanlines overlay on Wii U, then verify that only the game area is covered.