doc: Minor corrections to installation instructions.
glretrace: Create ES1 contexts only w/ GLX_EXT_create_context_es2_profile.

Unfortunately certain versions of Mesa only advertise
GLX_EXT_create_context_es2_profile.
glretrace: Retry without GLX_CONTEXT_DEBUG_BIT_ARB.

To workaround Mesa.
glstate: Don't dump GL_DEBUG_CALLBACK_*.

These parameters are overwritten by glretrace, so dumping them is a
distraction.
gui: Update www.opengl.org URLs.

Avoid the redirection.