Restore SDL context before VSync updates

[themuffinator]

Summary

• make the SDL OpenGL context current before every centralized swap-interval update
• cover startup, display-mode changes, and live r_swapInterval edits through the existing helper
• extend the Linux VSync and shared SDL3 backend contracts to lock in ordering
• document the native-Wayland compatibility fix for 0.12.0

Investigation

A native SDL Wayland run performed while investigating #89 logged:

SDL3: failed to set swap interval 0: No OpenGL context has been made current

The initial window-parameter application may leave SDL's GL context detached before startup applies VSync. GLimp_SetScreenParms already repairs the context on its own path, but SDL3_ApplySwapInterval is also called directly during startup and live cvar handling. Moving the guard into that central helper makes every caller safe and leaves the existing error/accounting path intact.

This corrects a concrete initialization error found during #89 investigation. It does not close #89: the reporter's CachyOS/RTX 3070 performance trace is still needed for the broader settings-panel claim.

Validation

• python tools/tests/linux_vsync_support.py
• python tools/tests/macos_sdl3_backend_guard.py
• python tools/tests/linux_sdl3_glew_loader.py
• python tools/tests/sdl3_multidisplay_windowing.py
• tools/build/meson_setup.ps1 compile -C builddir renderer-gl_x64
• python -m py_compile tools/tests/linux_vsync_support.py tools/tests/macos_sdl3_backend_guard.py
• git diff --check origin/main...HEAD

No external code was incorporated.

Related: #89

[themuffinator]

Native Wayland runtime evidence is now available from the Clang/no-libdecor commit job in run 33015438667.

I downloaded commit-linux-wayland-x64-clang-no-libdecor-wayland-only-renderer-validation and checked the complete artifact:

• all six reported validation cases passed (default safety, window lifecycle, repeated window transitions, mouse capture, mouse-capture stress, and display diagnostics)
• SDL3: swap interval set to 0 appears 36 times across startup and transition runs
• failed to set swap interval: 0 matches
• No OpenGL context has been made current: 0 matches
• renderer diagnostics report zero GL errors/fatals/error lines

This directly exercises native SDL Wayland under Mesa llvmpipe and confirms the initialization failure observed during #89 investigation is gone on this CI path. The wider real-hardware performance question in #89 remains open separately.