feat(render): Renderer with LOD selection, 3.3 and 4.3+ upload paths, headless smoke test

src/gl/loader.cpp

@@ -26,6 +26,7 @@ namespace plot::gl {
 // The one definition of the entry point table declared in api.hpp.
 #define PLOT_GL_DEF(ret, name, args) ret (*name) args = nullptr;
 PLOT_GL_FUNCS(PLOT_GL_DEF)
+PLOT_GL_OPTIONAL_FUNCS(PLOT_GL_DEF)
 #undef PLOT_GL_DEF
 
 namespace {
@@ -171,6 +172,11 @@ bool load(ProcLoader loader) noexcept {
 
   PLOT_GL_FUNCS(PLOT_GL_RESOLVE)
 
+  // Above-baseline entries: resolved the same way, but a name the driver does not
+  // have leaves its pointer null instead of failing the load, so a 3.3-only
+  // context keeps working. Callers gate them on caps() (see api.hpp).
+  PLOT_GL_OPTIONAL_FUNCS(PLOT_GL_RESOLVE)
+
 #undef PLOT_GL_RESOLVE
 
   // The table may have changed, so whatever caps() cached for the old one is