glad OpenGL loader generator

[Justin Whear]

It looks like your D backend generates string literals with literal null 
bytes--this is unnecessary as string literals are already null 
terminated. (Documented here: http://dlang.org/interfaceToC.html (see 
final bullet under "Call­ing C Func­tions")

Also, it looks like loading each extension requires scanning all 
available extension names, e.g. loading 25 extensions requires 25 linear 
scans of the extension list.  You might consider caching and sorting if 
the number of extensions requested is more than a handful.