Add portable runtime VSync control

[relh]

Summary

• expose a consistent runtime window.vsync getter/setter on Linux, Windows, and macOS
• apply swap-interval changes to the correct current native OpenGL context
• report unsupported/erroring GLX controls instead of recording a state change that did not happen
• compile-check the public property API on Windy's existing Linux/Windows/macOS CI matrix

Validation

• nim check --hints:off tests/test.nim
• nim check --hints:off --os:windows tests/test.nim
• nim check --hints:off --os:macosx tests/test.nim
• nim check --hints:off -d:emscripten tests/test.nim
• nim c --hints:off tests/test.nim
• nim c --hints:off examples/basic.nim
• runtime enable/disable/re-enable proof under Xvfb

This keeps the constructor option intact while allowing applications to temporarily disable compositor-blocking swaps (for example while occluded) without recreating their window.