Remove the headless OpenGL display mutex workaround

[ciscorn]

MapLibre Native has a thread-safety issue in headless GL display create/teardown, so maplibre-native-rs adds a mutex as a workaround (headlessDisplayMutex in src/cpp/map_renderer.h).

Once the fix (maplibre/maplibre-native#4332) is merged (and MLN_COMMIT and release in Cargo.toml are bumped), the workaround is no longer needed and can be removed.

Upstream Fix (Blocker)

• fix(core): make headless OpenGL display singletons thread-safe maplibre-native#4332

Related

• feat: support OpenGL EGL and GLX contexts #230
• Support Linux OpenGL rendering via EGL #229