[Davey-Hughes]

Moves the VR mode from #22 onto the unified VR view API in libretro/RetroArch#19811 (one OpenXR core for the desktop and Quest headsets, with head tracking).

It needs that pull request's libretro.h, so it should merge after it.

Against that RetroArch the current VR mode never starts. It negotiates once in retro_load_game() and waits for a recommended eye size, which the frontend only fills in while a headset session is running. Its map also lacks RETRO_VIDEO_VIEWS_FLAG_TRACKED, so it would show as a flat stereo screen.

What changes:

• The core asks GET_VIDEO_VIEWS_STATUS every frame, enters VR on the first frame with RETRO_VIDEO_VIEWS_STATUS_HMD, sends a TRACKED map at the recommended eye size, and goes flat with an empty map.
• Entering sets the engine's screen to one eye and reports the 2x-wide frame with SET_SYSTEM_AV_INFO. Leaving restores the flat size. A new eye size, flagged by TARGET_RESIZED or written back by the map, is followed, and the eye targets are rebuilt.
• VR needs Color Format 24-bit (Standard). SET_PIXEL_FORMAT is only legal at load, so VR no longer switches HDR off at run time. On another format the core says so once when a headset is present.
• A video reinit restarts the headset session, so a headset that goes missing is waited for about five seconds, repeating the last frame, before the core goes flat.
• Supersampling is ignored while VR is active instead of being turned off, so doom_hdr_ssaa keeps the user's choice.

Commits:

1. libretro-common: update libretro.h for the unified VR view API: the header from OpenXR: one core for the desktop and Quest headsets, with head tracking, controllers, settings and desktop OpenGL RetroArch#19811, taken whole.
2. libretro: follow the headset through GET_VIDEO_VIEWS_STATUS: the change itself.

To try it:

1. Build with HAVE_VR=1.
2. RetroArch built from OpenXR: one core for the desktop and Quest headsets, with head tracking, controllers, settings and desktop OpenGL RetroArch#19811 (./configure --enable-openxr on Linux, or the Quest build), with an OpenGL video driver. glcore is the one tested here.
3. Settings > Video > Stereo 3D & Screens > Headset Output (OpenXR) on, and boom3_vr_enable = enabled.

Tested on Linux, against a build of that pull request:

• Monado's simulated headset, glcore driver, with the Doom 3 demo: the core enters VR at the runtime's eye size, and both eyes reach the headset as a tracked projection.
• make -C neo HAVE_VR=1 builds at each commit. libretro.cpp also compiles without HAVE_VR.

@cscd98 tested this branch on a Quest, and it works there.

Known, and not addressed here:

• Two defects in the core's existing VR drawing, seen on the Frame and reproduced under Monado: a GUI monitor scene in the Mars City intro draws grey in VR, and the dropship's exhaust effect shows in one eye only.
• The two known bugs listed in Libretro VR support #22.
• Full-size eyes are heavy: 13 to 36 ms a frame on the Frame.