Split constants out into their own file. Hide the
values in named types in a struct, so that when
using the gldebug tag we can attach extra
information (such as the name of a uniform and
attribute).
LGTM=nigeltao
R=golang-codereviews, bryanturley, nigeltao
CC=davidday, golang-codereviews
https://golang.org/cl/137630043
David Crawshaw