Fix wgpu integration example

[thenlevy]

On master, I can make the integration example crash on XFCE by resizing the window quickly. I believe this to be due to the fact that the window's size can change between the last time we processed the Resize event, and the moment we generate the next frame texture.

This PR avoids this by checking that the last processed size is correct before redrawing.

[hecrj]

What kind of crash? Do you get a backtrace?

I think we should probably make all of our drawing operations return a Result and fail gracefully.

[thenlevy]

It is a set_scissor_rect error probably due to a mismatch between the Viewport's and the frame's dimensions. Here is a backtrace

[2021-12-09T13:21:42Z ERROR wgpu::backend::direct] Handling wgpu errors as fatal by default
thread 'main' panicked at 'wgpu error: Validation Error

Caused by:
    In a RenderPass
      note: encoder = `<CommandBuffer-(0, 51, Vulkan)>`
    In a set_scissor_rect command
    Invalid ScissorRect parameters

', /home/nlevy/.cargo/registry/src/github.com-1ecc6299db9ec823/wgpu-0.11.1/src/backend/direct.rs:2195:5
stack backtrace:
   0: rust_begin_unwind
             at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/std/src/panicking.rs:517:5
   1: std::panicking::begin_panic_fmt
             at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/std/src/panicking.rs:460:5
   2: wgpu::backend::direct::default_error_handler
             at /home/nlevy/.cargo/registry/src/github.com-1ecc6299db9ec823/wgpu-0.11.1/src/backend/direct.rs:2195:5
   3: core::ops::function::Fn::call
             at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/ops/function.rs:70:5
   4: <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call
             at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/alloc/src/boxed.rs:1650:9
   5: wgpu::backend::direct::ErrorSinkRaw::handle_error
             at /home/nlevy/.cargo/registry/src/github.com-1ecc6299db9ec823/wgpu-0.11.1/src/backend/direct.rs:2183:9
   6: wgpu::backend::direct::Context::handle_error
             at /home/nlevy/.cargo/registry/src/github.com-1ecc6299db9ec823/wgpu-0.11.1/src/backend/direct.rs:184:9
   7: <wgpu::backend::direct::Context as wgpu::Context>::command_encoder_end_render_pass
             at /home/nlevy/.cargo/registry/src/github.com-1ecc6299db9ec823/wgpu-0.11.1/src/backend/direct.rs:1957:13
   8: <wgpu::RenderPass as core::ops::drop::Drop>::drop
             at /home/nlevy/.cargo/registry/src/github.com-1ecc6299db9ec823/wgpu-0.11.1/src/lib.rs:2823:13
   9: core::ptr::drop_in_place<wgpu::RenderPass>
             at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/ptr/mod.rs:188:1
  10: iced_wgpu::quad::Pipeline::draw
             at ./wgpu/src/quad.rs:246:13
  11: iced_wgpu::backend::Backend::flush
             at ./wgpu/src/backend.rs:125:13
  12: iced_wgpu::backend::Backend::present
             at ./wgpu/src/backend.rs:89:13
  13: integration_wgpu::main::{{closure}}::{{closure}}
             at ./examples/integration_wgpu/src/main.rs:199:29
  14: iced_graphics::renderer::Renderer<B>::with_primitives
             at ./graphics/src/renderer.rs:40:9
  15: integration_wgpu::main::{{closure}}
             at ./examples/integration_wgpu/src/main.rs:198:25
  16: winit::platform_impl::platform::sticky_exit_callback
             at /home/nlevy/.cargo/git/checkouts/winit-57d3141eaf559308/1e6623c/src/platform_impl/linux/mod.rs:753:5
  17: winit::platform_impl::platform::x11::EventLoop<T>::run_return
             at /home/nlevy/.cargo/git/checkouts/winit-57d3141eaf559308/1e6623c/src/platform_impl/linux/x11/mod.rs:310:21
  18: winit::platform_impl::platform::x11::EventLoop<T>::run
             at /home/nlevy/.cargo/git/checkouts/winit-57d3141eaf559308/1e6623c/src/platform_impl/linux/x11/mod.rs:388:9
  19: winit::platform_impl::platform::EventLoop<T>::run
             at /home/nlevy/.cargo/git/checkouts/winit-57d3141eaf559308/1e6623c/src/platform_impl/linux/mod.rs:669:56
  20: winit::event_loop::EventLoop<T>::run
             at /home/nlevy/.cargo/git/checkouts/winit-57d3141eaf559308/1e6623c/src/event_loop.rs:154:9
  21: integration_wgpu::main
             at ./examples/integration_wgpu/src/main.rs:102:5
  22: core::ops::function::FnOnce::call_once
             at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

The problem is that, AFAIK it is not possible to get a Result when trying to draw with wgpu. It is however possible to set a different error handler for validation errors.