Load GLX dynamically with GLAD

This is accomplished through the addition of a GLAD GLX loader in the
`thirdparty` directory.

This is another step towards a nice Wayland/X11 interoperation.

platform/linuxbsd/detect.py

@@ -356,7 +356,6 @@ def configure(env: "Environment"):
 
     if env["opengl3"]:
         env.Append(CPPDEFINES=["GLES3_ENABLED"])
-        env.ParseConfig("pkg-config gl --cflags --libs")
 
     env.Append(LIBS=["pthread"])