Make anti-aliasing best-effort rather than panicking.
Make anti-aliasing best-effort rather than panicking.

Here's an example of the panic it fixes:
aij@yhu:~/git/piston-examples/deform$ RUST_BACKTRACE=1 cargo run
   Compiling piston-examples-deform v0.0.1 (file:///home/aij/git/piston-examples/deform)
     Running `target/debug/piston-examples-deform`
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thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: "Couldn\'t find matching GLX visual"', /home/aij/git/rust/src/libcore/result.rs:775
stack backtrace:
   1:     0x7f9a2618ee59 - sys::backtrace::write::hb00e07d997b04265HyD
   2:     0x7f9a261929e2 - panicking::on_panic::he2ac5bb2c4c90392ZOJ
   3:     0x7f9a26187853 - rt::unwind::begin_unwind_inner::hcc5be3c2c3b2436bGuJ
   4:     0x7f9a26187c11 - rt::unwind::begin_unwind_fmt::h2ce25b5d5abb3f6chtJ
   5:     0x7f9a26192427 - rust_begin_unwind
   6:     0x7f9a261c3904 - panicking::panic_fmt::hb16892aee821a66fH0A
   7:     0x7f9a2608c6cc - result::Result<T, E>::unwrap::h289021542769723922
                        at /home/aij/git/rust/src/libcore/macros.rs:27
   8:     0x7f9a2608b1c9 - Sdl2Window::new::hee2cbff16eba6f8eYaa
                        at /home/aij/.cargo/git/checkouts/sdl2_window-99fd7f1ca94d6f5b/master/src/lib.rs:77
   9:     0x7f9a260480b2 - main::hc0771fbefb648e99vaa
                        at src/main.rs:25
  10:     0x7f9a2619be18 - rust_try_inner
  11:     0x7f9a2619be05 - rust_try
  12:     0x7f9a2619447a - rt::lang_start::h66c15121e827c945sJJ
  13:     0x7f9a26048af4 - main
  14:     0x7f9a252cab44 - __libc_start_main
  15:     0x7f9a26047d98 - <unknown>
  16:                0x0 - <unknown>
An unknown error occurred

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PistonDevelopers/sdl2_window,bvssvni/sdl2_window,Potpourri/sdl2_window