Modal popups result in panic due to scissor rect problem#99

## Description When I create a modal popup, imgui crashes: ``` 2023-04-28T19:49:10.076Z 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, 1129, Vulkan)>` In a set_scissor_rect command Scissor Rect { x: 0, y: 0, w: 1602, h: 902 } is not contained in the render target Extent3d { width: 1600, height: 900, depth_or_array_layers: 1 } ``` ## Repro steps Run the demo example (and select the modal popup in the imgui demo window) ## System Information ```Toml imgui = { version = "0.11.0", features = ["docking"] } imgui-sys = { version = "0.11.0", features = ["docking"] } imgui-wgpu = { git = "https://github.com/Yatekii/imgui-wgpu-rs" } imgui-winit-support = "0.11.0" ```