Create OpenGL 3.2 Core context on Linux
Create OpenGL 3.2 Core context on Linux

This is mainly to ensure that Halide works correctly with modern OpenGL, in
particular in applications that don't provide compatibility contexts.


Former-commit-id: 8b3f6f6a41614f9e5b88f9ccc8f2b8501732d938
darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide