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

include/plot/gl/loader.hpp

@@ -0,0 +1,28 @@
+#pragma once
+#include <plot/gl/api.hpp>
+
+namespace plot::gl {
+
+/// Entry point resolver supplied by the caller, normally the *GetProcAddress
+/// of the API that owns the current context: `eglGetProcAddress`,
+/// `glXGetProcAddressARB` or `wglGetProcAddress`.
+using ProcLoader = void* (*)(const char*);
+
+/// Resolves every entry point of `PLOT_GL_FUNCS` (see api.hpp) into the global
+/// table declared there, so the rest of the library can call `gl*` directly.
+///
+/// Returns false when any required name cannot be found; entries that did
+/// resolve keep their pointers, so a failed load() never breaks a table that
+/// was already usable. `loader` is consulted first; when it is null, or returns
+/// null for a name, the fallback is `dlsym(RTLD_DEFAULT, name)` and then
+/// `dlopen("libGL.so.1")` + `dlsym`. The fallback makes the library usable
+/// without a caller-supplied resolver, but only a context-aware resolver is
+/// guaranteed to return pointers valid for the *current* context — pass one
+/// whenever a context is current.
+///
+/// A load() attempt invalidates the caps() cache. The table is process-global
+/// and is not synchronised, so call from the thread that owns the current
+/// context.
+bool load(ProcLoader loader = nullptr) noexcept;
+
+}  // namespace plot::gl