Fixed runtime error at GL context creation

[aumouvantsillage]

When running in Fedora 39, GUI initialization failed with the backtrace below.
This pull request fixes the issue by providing a valid Interface instead of None in DirectContext::new_gl(...).

This solution is based on an example from the rust-skia repository.

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', loopers-gui/src/skia.rs:90:60
stack backtrace:
   0: rust_begin_unwind
             at /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/std/src/panicking.rs:593:5
   1: core::panicking::panic_fmt
             at /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/panicking.rs:67:14
   2: core::panicking::panic
             at /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/panicking.rs:117:5
   3: core::option::Option<T>::unwrap
             at /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/option.rs:935:21
   4: loopers_gui::skia::skia_main
             at ./loopers-gui/src/skia.rs:90:26
   5: loopers_gui::Gui::start
             at ./loopers-gui/src/lib.rs:208:9
   6: loopers::loopers_jack::jack_main
             at ./loopers/src/loopers_jack.rs:314:9
   7: loopers::main
             at ./loopers/src/main.rs:139:13
   8: core::ops::function::FnOnce::call_once
             at /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/ops/function.rs:250:5