fix(flatpak): NVIDIA Wayland Zink override in entrypoint (#713)

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Description

Easy upstream port of Snapmaker/OrcaSlicer#713 (OPEN): fix blank Flatpak Prepare/Preview canvases on NVIDIA Wayland by mirroring the AppImage Zink policy in the Flatpak entrypoint.

The AppImage launcher in src/dev-utils/platform/unix/build_linux_image.sh.in already routes OpenGL through Mesa Zink for NVIDIA drivers newer than 555 under Wayland. That workaround was never applied to Flatpak. glxinfo / nvidia-smi are not available in the Flatpak runtime, so the entrypoint now derives the driver major from the mounted org.freedesktop.Platform.GL.nvidia-* extension in /.flatpak-info.

Zink env flags (same controls as the AppImage launcher):

• ZINK_DISABLE_OVERRIDE=1 — skip the Mesa/Zink override even on affected NVIDIA Wayland sessions.
• ZINK_FORCE_OVERRIDE=1 — force Mesa GLX/EGL + Zink on Wayland even when the parsed driver major is missing or ≤555.

Activation (NVIDIA Flatpak + Wayland only): if not disabled, and (ZINK_FORCE_OVERRIDE=1 or driver major >555), set __GLX_VENDOR_LIBRARY_NAME=mesa, __EGL_VENDOR_LIBRARY_FILENAMES to the Mesa EGL vendor JSON, MESA_LOADER_DRIVER_OVERRIDE=zink, and GALLIUM_DRIVER=zink. The existing WebKit DMA-BUF disable (WEBKIT_DISABLE_DMABUF_RENDERER=1) is kept for all NVIDIA Flatpak sessions.

Non-NVIDIA and X11 sessions are unchanged. This PR does not touch GLEW/GLAD. Snapmaker/OrcaSlicer#351 is a later Hard port that migrates GLEW → GLAD; once that lands, native NVIDIA GL works and this Zink workaround should become opt-in (ZINK_FORCE_OVERRIDE=1 only).

+23/−2 in scripts/flatpak/entrypoint only. Ultra binary path remains /app/bin/snapmaker-orca.

Screenshots/Recordings/Graphs

N/A — Flatpak launcher env policy only; no UI code changes.

Tests

• ShellCheck 0.11.0 (-s sh) on scripts/flatpak/entrypoint: clean.
• Local POSIX activation-condition tests: driver-major extraction (610 / 555 / 470), Wayland vs X11, threshold >555, ZINK_DISABLE_OVERRIDE=1 wins over force, ZINK_FORCE_OVERRIDE=1 on old/empty major, WebKit DMA-BUF retain, Ultra exec path preserved.
• Cloud cannot smoke Flatpak NVIDIA Wayland. There is no Flatpak runtime, NVIDIA driver, or Wayland session in this environment. Hardware verification remains: install the Flatpak on NVIDIA Wayland (driver >555), confirm Prepare/Preview render, and confirm ZINK_DISABLE_OVERRIDE=1 / ZINK_FORCE_OVERRIDE=1 behave as documented. Upstream Fix blank Flatpak canvas on NVIDIA Wayland Snapmaker/OrcaSlicer#713 was reported working on CachyOS KDE Wayland + RTX 3090 / 4070S, driver 610.57.04.

  

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Superseded by #8: its Flatpak entrypoint carries this rule (NVIDIA Wayland, driver newer than 555) behind ZINK_AUTO_NVIDIA=1, with the same ZINK_FORCE_OVERRIDE / ZINK_DISABLE_OVERRIDE switches, and native GL through GLAD as the default.