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

tests/egl_headless.hpp

@@ -0,0 +1,57 @@
+#pragma once
+#include <plot/core/status.hpp>
+#include <plot/core/types.hpp>
+#include <plot/gl/handle.hpp>
+
+#include <cstdint>
+
+namespace plottest {
+
+using plot::u32;
+
+/// A surfaceless EGL context plus an off-screen RGBA framebuffer, so renderer
+/// output can be asserted pixel by pixel without a window or a display server.
+///
+/// Test-only: `plot_core` neither links nor includes EGL.
+class HeadlessContext {
+ public:
+  explicit HeadlessContext(u32 w = 256, u32 h = 256) noexcept;
+  ~HeadlessContext();
+
+  HeadlessContext(const HeadlessContext&) = delete;
+  HeadlessContext& operator=(const HeadlessContext&) = delete;
+
+  /// Makes an OpenGL 3.3 core context current on this thread, resolves the GL
+  /// entry points against it (`plot::gl::load(eglGetProcAddress)`) and
+  /// allocates the colour attachment. Idempotent. On failure `st` (when
+  /// non-null) carries the reason, every resource already created is released,
+  /// and the error is also printed to stderr so a failing CTest run is
+  /// diagnosable.
+  bool create(plot::Status* st = nullptr) noexcept;
+
+  /// Releases the framebuffer, the GL context and the EGL display. Safe to call
+  /// more than once, and to call after a failed create().
+  void destroy() noexcept;
+
+  /// Binds the off-screen target and reads one RGBA8 pixel, in GL's
+  /// origin-at-the-bottom-left convention. False when it cannot be read.
+  bool read_pixel(u32 x, u32 y, std::uint8_t rgba[4]) const noexcept;
+
+  u32 width = 0;
+  u32 height = 0;
+  /// The headless path create() selected ("egl surfaceless" or "egl pbuffer");
+  /// empty before a successful create() and after destroy(). Setting
+  /// PLOT_HEADLESS_FORCE_PBUFFER in the environment skips the surfaceless
+  /// attempt, so the pbuffer fallback can be exercised on a driver that does
+  /// support surfaceless contexts.
+  const char* egl_path = "";
+
+ private:
+  plot::gl::Texture color_;
+  plot::gl::Framebuffer framebuffer_;
+  void* display_ = nullptr;  // EGLDisplay
+  void* context_ = nullptr;  // EGLContext
+  void* surface_ = nullptr;  // EGLSurface, null on the surfaceless path
+};
+
+}  // namespace plottest