fix: add NVIDIA GBM EGL platform fallback

[miaulightouch]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Add a startup fallback for the NVIDIA GBM EGL external platform manifest.

When an NVIDIA GPU is detected, init-video now checks for an existing NVIDIA GBM EGL external platform JSON under:

• /usr/share/egl/egl_external_platform.d
• /etc/egl/egl_external_platform.d

If none is found, it creates:

/etc/egl/egl_external_platform.d/15_nvidia_gbm.json

with libnvidia-egl-gbm.so.1 as the ICD library.

Benefits of this PR and context:

Some NVIDIA container environments provide the required driver libraries but do not install the GBM EGL external platform manifest. Without this JSON file, EGL/Wayland GBM initialization can fail even when the NVIDIA GBM library is available.

This follows the existing init-video pattern of creating missing NVIDIA runtime config files under /etc only when they are absent.

How Has This Been Tested?

• Tested with NVIDIA GPU containers where json file was missing
• Verified EGL functionality after automatic setup (eglinfo -B)

Source / References:

https://github.com/selkies-project/docker-selkies-egl-desktop/blob/8dd03fba2a33ac774d61f46375a19ed1d9c5b51a/Dockerfile#L248-L264