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

include/plot/gl/caps.hpp

@@ -0,0 +1,30 @@
+#pragma once
+#include <plot/gl/api.hpp>
+
+namespace plot::gl {
+
+/// What the current context can do. Only OpenGL 3.3 core is required; every
+/// field above that baseline is false when the driver does not offer it.
+struct Caps {
+  int major = 0;
+  int minor = 0;
+  /// GL_VERSION / GL_RENDERER / GL_VENDOR strings. They are owned by the
+  /// context: valid until it is destroyed, and empty ("") when nothing has
+  /// been probed yet.
+  const char* renderer = "";
+  const char* vendor = "";
+  bool compute = false;         ///< GL 4.3+ (compute shaders)
+  bool buffer_storage = false;  ///< GL 4.4+ or ARB_buffer_storage
+  bool persistent_map = false;  ///< GL 4.4+ or ARB_buffer_storage
+  bool core_profile = false;    ///< GL_CONTEXT_CORE_PROFILE_BIT
+};
+
+/// Capabilities of the context that is current on the calling thread, probed
+/// once on the first successful query and cached afterwards. Safe to call
+/// before load(): with no usable context (or no resolved entry points) every
+/// field stays zero. `load()` invalidates the cache. The cache is
+/// process-global and unsynchronised: probe from the thread that owns the
+/// current context.
+const Caps& caps() noexcept;
+
+}  // namespace plot::gl