gl/gl.h should not include gl/Texture.h

For some reason Xcode doesn’t complain about this, but if you try to
compile directly with clang (I guess the compiler flags I’m using must
be different) you’ll get duplicated symbols as Texture.h also include
gl.h
Minor include issue in Sphere.h
Revert "Minor include issue in Sphere.h"

This reverts commit ffa939d26620a2ea134e01f6d7eb47f322a83b88.
Revert ffa939d..e5bd2ae

This rolls back to commit ffa939d26620a2ea134e01f6d7eb47f322a83b88.
Merge pull request #622 from simongeilfus/glNext_minor_include_fixes

gl/gl.h should not include gl/Texture.h