Simplify now that toktx no longer needs desktop GL header to build.
Note this commit is incomplete. Remainder plus explanation will be
in next commit.
Simplify since toktx no longer needs desktop GL header to build. When I tried
to commit this with libktx/CMakeLists.txt git got confused and thought I
was renaming libktx/CMakeLists.txt to CMakeLists.txt. So a separate commit.