feat(gl): dependency-free GL loader, RAII handles, caps probe, headless EGL harness

tests/CMakeLists.txt

@@ -1,8 +1,34 @@
+# EGL powers the surfaceless headless context the GL tests assert pixels
+# against. It is a *test-only* dependency: plot_core never links or includes it,
+# so a machine without EGL can still build the library (-DPLOT_BUILD_TESTS=OFF).
+# Prefer CMake's own module, fall back to pkg-config, and fail loudly if neither
+# can supply EGL rather than silently dropping the GL tests.
+find_package(EGL QUIET)
+if(NOT EGL_FOUND)
+  find_package(PkgConfig QUIET)
+  if(PkgConfig_FOUND)
+    pkg_check_modules(EGL_PC QUIET IMPORTED_TARGET egl)
+  endif()
+endif()
+
+if(TARGET EGL::EGL)
+  set(plot_egl_library EGL::EGL)
+elseif(TARGET PkgConfig::EGL_PC)
+  set(plot_egl_library PkgConfig::EGL_PC)
+else()
+  message(FATAL_ERROR
+    "plot: PLOT_BUILD_TESTS=ON requires EGL (EGL/egl.h plus libEGL) for the headless "
+    "OpenGL context used by the GL tests. Install it (e.g. libegl-dev, libegl1-mesa-dev "
+    "or mesa-libEGL-devel) or configure with -DPLOT_BUILD_TESTS=OFF.")
+endif()
+
 add_executable(plot_tests
   main.cpp
   test_transform.cpp
+  test_gl_context.cpp
+  egl_headless.cpp
 )
-target_link_libraries(plot_tests PRIVATE plot::core)
+target_link_libraries(plot_tests PRIVATE plot::core ${plot_egl_library})
 plot_apply_warnings(plot_tests)
 
 add_test(NAME plot_tests COMMAND plot_tests)