Fix ExternalTexture_OpenGL throw-stubs to avoid MSVC C4702 under /WX

[bkaradzic-microsoft]

Fixes MSVC C4702 (unreachable code) under /WX in Plugins/ExternalTexture/Source/ExternalTexture_OpenGL.cpp so the OpenGL TU builds cleanly without the project-wide /wd4702 workaround.

What changed

Plugins/ExternalTexture/Source/ExternalTexture_OpenGL.cpp keeps the unimplemented Impl stubs as throw std::runtime_error{"not implemented"} (so callers get an unambiguous diagnostic matching every other backend's unimplemented path) and wraps the #include "ExternalTexture_Shared.h" line in #pragma warning(push)/(disable: 4702)/(pop). The shared dispatchers instantiated by that include unconditionally call the stubs, so MSVC's flow analysis flags the dispatcher's post-call code as unreachable; the pragma suppresses C4702 only for the dispatcher code instantiated in this translation unit. Any other code in this file is still subject to /WX C4702 enforcement.

Plugins/ExternalTexture/CMakeLists.txt - drop the /wd4702 target compile option, which previously silenced C4702 across the whole target.

Alternatives considered

• [[noreturn]] on the stubs: MSVC propagates "never returns" through the shared dispatcher, which flags more post-call statements as unreachable. Tried and rejected (it made the warning worse).
• TU-wide /wd4702 via target_compile_options: silences any future legitimate C4702 elsewhere in the same TU, defeating /WX. The current localized #pragma warning(push/pop) block keeps the rest of the TU under /WX enforcement.
• Replacing the throws with inert return-default stubs: changes product behaviour (callers would silently receive a null texture instead of a clear "not implemented" error) to work around a compiler warning. Rejected on principle.

Verified

Clean under Debug + Release + RelWithDebInfo on OpenGL (win32-gl, GRAPHICS_API=OpenGLWindowsDevOnly) and D3D11 (win32).

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Landing context

This PR is one of 7 splits from the proven CI-green combined preview in draft PR #1702 (see #1702 for the full intended end-state and verified CI run 26044922430).

Note: the original split included an 8th PR (#1709, ES2020+ -> ES2019 syntax-repair polyfill for Chakra). It was closed in favour of investigating @babel/standalone properly (#1711).

Recommended landing order

Tier 1 - parallel-reviewable, no source conflicts:

1. Fix ExternalTexture_OpenGL throw-stubs to avoid MSVC C4702 under /WX #1703 - ExternalTexture C4702 build fix
2. Document accurate root cause for post-#1695 pixel-diff fallouts #1704 - config.json reason rewrites (5 entries)
3. Document accurate root cause for 17 subtle pixel-diff tests #1705 - config.json reason rewrites (17 entries)

Tier 2 - sequential, each touches Apps/Playground/CMakeLists.txt SCRIPTS list + Apps/Playground/Shared/AppContext.cpp LoadScript order; rebase the next branch after the previous merges:

4. Bump JsRuntimeHost for File/FileReader polyfill (re-enables 19 GLTF/OBJ tests) #1706 - File/Blob/FileReader polyfill (largest test impact: 19 re-enables)
5. Add fetch() polyfill over XMLHttpRequest for Playground #1707 - fetch polyfill
6. Add DOM globals polyfill + native AbortController for Playground #1708 - DOM globals + native AbortController + Android CMake link
7. Add cubemap auto-expand polyfill for Playground (re-enables 7 PBR tests) #1710 - Cubemap auto-expand polyfill (loaded after babylon.max.js)

Reference policy reminder

Reference PNGs across all 7 PRs come from Babylon.js; never re-baked by BN. Combined diff: 0 PNGs.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Replaces unimplemented throw stubs in the OpenGL ExternalTexture backend with default-value returns to silence MSVC C4702 (unreachable code) warnings under /WX, and removes the corresponding /wd4702 workaround.

Changes:

• Replace throw std::runtime_error{"not implemented"} with default returns / empty bodies in ExternalTexture_OpenGL.cpp.
• Remove the OpenGL+MSVC-specific /wd4702 compile option from CMakeLists.txt.
• Add an explanatory comment describing the rationale for the stub behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
Plugins/ExternalTexture/Source/ExternalTexture_OpenGL.cpp	Convert throwing stubs into inert default-returning stubs and add an explanatory comment.
Plugins/ExternalTexture/CMakeLists.txt	Remove the now-unneeded /wd4702 MSVC workaround for the OpenGL build.

[ryantrem]

The PR description seems out of date with the changes. Is it just moving the warning suppression into the code to make it more restrictive (only part of the file, not the whole file)?

[bghgary]

[Reviewed by Copilot on behalf of @bghgary]

LGTM barring issues already mentioned, plus one inline comment.