Approximately three or four times now, the issue of pointers being
in an inconsistent state been an issue in the video backend renderers
with regards to tripping up other developers.
Global (ugh) resources are put into a unique_ptr and will always have a
well-defined state of being - null or not null
This removes some nonsense in the extension loader where under an ES context we would still pull all function pointers and just continue onward if we
fail to pull one.
Now function pointers are only pulled if the version of GL or ES actually supports that function.
