Add native Wayland GL3 backend

[Third-Thing]

Summary

• Add a native Wayland platform backend with EGL and OpenGL 3 rendering.
• Add CMake integration for RMLUI_BACKEND=Wayland_GL3, including Wayland protocol generation.
• Add keyboard input, pointer input, cursors, window resize/close handling, and key repeat.
• Document Wayland_GL3 in the backend tables.

Known limitations

• Clipboard support is process-local only, not integrated with the Wayland selection protocol.
• High DPI/output scale handling is not implemented yet.
• Touch input is not implemented.
• Scroll handling currently uses basic axis values, not axis_value120.

Clipboard support will be addressed in the next PR.
No high DPI or touch screen is available for testing, so those are left to concerned parties.

Testing

• Configured with RMLUI_BACKEND=Wayland_GL3.
• Built Wayland backend and samples.
• Manually tested keyboard input, modifiers, Caps Lock, key repeat, and focus-loss behavior.
• Manually tested mouse clicks, drags, scrolls, hover states and shape changes.
• Manually tested window resizing, minimizing, closing, and multi-desktop pinning.

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Tested on:

openSUSE Leap 16
KDE Plasma 6.4.2 (Wayland)
i5-4690K + NVIDIA GTX 1050 ti

[Third-Thing]

The build failures appear to be related to existing code relative to changes in the CI environment (macos-15-arm64 → macos-26-arm64), not this PR's changes.

• AGL/agl.h is missing on the newer macOS/Xcode SDK. That is existing GL2 macOS shell code, unrelated to Wayland.
• Homebrew behavior changed: on the older run, brew install sdl2 installed actual sdl2 and CMake found SDL2_image. On the new run, Homebrew installs sdl2-compat plus sdl3, so CMake detects SDL3 and then fails because the workflow installed sdl2_image, not sdl3_image.