EXPORT -> WEAK in opengl backend.
EXPORT -> WEAK in opengl backend.


Former-commit-id: 6511a7e618bf096650fb8a426b62e90432d9e7e1
darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide