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

CMakeLists.txt

@@ -7,6 +7,8 @@ include(plot_options)
 add_library(plot_core
   src/core/status.cpp
   src/math/transform.cpp
+  src/gl/loader.cpp
+  src/gl/handle.cpp
 )
 add_library(plot::core ALIAS plot_core)