Allow statically-linked compilation by disabling feature flags

[zinifier]

Currently, trying to compile to x86_64-unknown-linux-musl (for portable binaries) produces a NoWaylandLib error and refuses to start:

$ RUST_BACKTRACE=full cargo run --release --target=x86_64-unknown-linux-musl
    Finished `release` profile [optimized] target(s) in 0.38s
     Running `/prog/.cargo/target/x86_64-unknown-linux-musl/release/sticker-printer-gui`
thread 'main' panicked at /prog/.cargo/git/checkouts/libcosmic-41009aea1d72760b/75a11b3/iced/winit/src/program.rs:177:39:
Create event loop: Os(OsError { line: 63, file: "/prog/.cargo/git/checkouts/winit-d03127a0b2faf55d/1cc02bd/src/platform_impl/linux/wayland/event_loop/mod.rs", error: NoWaylandLib })
stack backtrace:
   0:     0x763043850cda - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h4d925a43945ad333
   1:     0x76304389ceb3 - core::fmt::write::hcfaeabde0feb60be
   2:     0x76304384bdc3 - std::io::Write::write_fmt::h4ea182833dad27bd
   3:     0x763043850b22 - std::sys::backtrace::BacktraceLock::print::h378b293f8d5bee0e
   4:     0x763043851d1d - std::panicking::default_hook::{{closure}}::h55e037cd46ef863d
   5:     0x763043851b62 - std::panicking::default_hook::h5b88978f11f489c4
   6:     0x7630438523e7 - std::panicking::rust_panic_with_hook::h2c5057fbc1c11ffd
   7:     0x76304385218a - std::panicking::begin_panic_handler::{{closure}}::h7279d07160b25030
   8:     0x7630438511a9 - std::sys::backtrace::__rust_end_short_backtrace::h01262d840bed71d8
   9:     0x763043851e1d - rust_begin_unwind
  10:     0x763042d16070 - core::panicking::panic_fmt::h186872714c27c18d
  11:     0x763042d164a6 - core::result::unwrap_failed::h265ca1dbdb4601a6
  12:     0x763042d945a3 - iced_winit::program::run::h124582e31af565f4
  13:     0x763042d38d1b - cosmic::app::run::h314103374714143f
  14:     0x763042de3377 - sticker_printer_gui::main::h8d6f5214e0e39217
  15:     0x763042da6f83 - std::sys::backtrace::__rust_begin_short_backtrace::h4ebc646de044798c
  16:     0x763042e016f9 - std::rt::lang_start::{{closure}}::ha460436e810f83e2
  17:     0x7630438441d7 - std::rt::lang_start_internal::h781f6455e6949f85
  18:     0x763042de3455 - main

That's because some dependencies enable some dynamic library features with a dlopen feature flag:

• smithay-clipboard/dlopen
• softbuffer/wayland-dlopen and softbuffer/x11-dlopen
• winit/wayland-dlopen
• maybe others?

I think most of these feature flags are enabled within cosmic-managed or forked repositories so this should be fixable!

It would be great to enable statically linked applications like Appimages using libcosmic! :)