Panic when using min_size on Window

[EbonJaeger]

Describe the bug
When using the min_size function when building a Window, the application panics with:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/emaddock/.cargo/git/checkouts/orbtk-90d798f349c2ab56/4479802/orbtk_tinyskia/src/tinyskia/mod.rs:50:63

To Reproduce
Steps to reproduce the behavior:

1. Generate a new project using the OrbTK template
2. Add a call to min_size(325.0, 500.0) when building the Window in main.rs
3. Run cargo run
4. See error

Expected behavior
The application should run and have a minimum size.

Desktop (please complete the following information):

• OS: Solus 4.3 Fortitude
• Browser: Vivaldi, though this isn't a web app.
• Rust version: 1.55.0

Additional context
Here is the full backtrace with cargo run:

    Finished dev [optimized + debuginfo] target(s) in 0.08s
     Running `target/debug/calculator`
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/emaddock/.cargo/git/checkouts/orbtk-90d798f349c2ab56/4479802/orbtk_tinyskia/src/tinyskia/mod.rs:50:63
stack backtrace:
   0: rust_begin_unwind
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/std/src/panicking.rs:515:5
   1: core::panicking::panic_fmt
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:92:14
   2: core::panicking::panic
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:50:5
   3: core::option::Option<T>::unwrap
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/option.rs:722:21
   4: orbtk_tinyskia::tinyskia::RenderContext2D::new
             at /home/emaddock/.cargo/git/checkouts/orbtk-90d798f349c2ab56/4479802/orbtk_tinyskia/src/tinyskia/mod.rs:50:22
   5: orbtk_orbclient::orbclient::window_builder::WindowBuilder<A>::build
             at /home/emaddock/.cargo/git/checkouts/orbtk-90d798f349c2ab56/4479802/orbtk_orbclient/src/orbclient/window_builder.rs:103:34
   6: orbtk::application::Application::window
             at /home/emaddock/.cargo/git/checkouts/orbtk-90d798f349c2ab56/4479802/orbtk/src/application.rs:71:9
   7: calculator::main
             at ./src/main.rs:10:5
   8: core::ops::function::FnOnce::call_once
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

main.rs:

use orbtk::prelude::*;

pub use self::main_state::*;
pub use self::main_view::*;

mod main_state;
mod main_view;

fn main() {
    Application::from_name("calculator")
        .window(move |ctx| {
            Window::new()
                .title("Calculator")
                .position((100.0, 100.0))
                .min_size(325.0, 500.0) // Added this line
                .size(325.0, 500.0)
                .resizeable(true)
                .child(MainView::new().title("Hello OrbTk").build(ctx))
                .build(ctx)
        })
        .run();
}

All other files in the project remain unchanged.