egl: preserve caller state and support surfaceless offscreen contexts

[tritao]

Summary

Make Coin's offscreen EGL handling safe for applications that already own EGL state and usable in surfaceless environments.

• Support explicit surfaceless EGL through `EGL_PLATFORM=surfaceless`.
• Reuse an active caller display when appropriate.
• Distinguish borrowed EGL displays from Coin-owned displays.
• Preserve and restore the client API, display, context, draw surface, and read surface.
• Support optional core-profile offscreen contexts through `COIN_EGL_CORE_PROFILE=1`.
• Add focused EGL context-preservation and surfaceless runtime coverage.

The old Wayland/X11 display-probing fallback is intentionally not restored. Coin honors an explicit surfaceless request, reuses active EGL state when possible, and otherwise follows the normal EGL default-display path.

Stack

Layer 2/18, based on PR #11.

[tritao]

Addressed the EGL review feedback:

• Integrated coverage into the existing Ubuntu job with Mesa EGL; no separate or core-only build.

• Scoped EGLBindingTest to Linux.

• Removed redundant void syntax and local constants provided by eglext.h.

• Added shared skip-code handling and coin_add_egl_test().

• Registered the test as egl;requires-egl.

• Split coverage into two clear cases:

  • Coin acquires and owns an explicitly requested surfaceless display.
  • Coin borrows an active caller display and restores its complete EGL binding.