Various versions of these headers cause harmless (but frustrating)
compile errors, e.g. *C.char being swapped out in some function
definitions for *C.uchar. To simplify builds, importing the
headers from khronos.org.
LGTM=nigeltao
R=golang-codereviews, nigeltao
CC=golang-codereviews
https://golang.org/cl/127020043
David Crawshaw