Backends, Examples: Added support for WebGPU and corresponding example (#3632)

(Squashed 11 commits)

backends/imgui_impl_wgpu.h

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+// dear imgui: Renderer for WebGPU
+// This needs to be used along with a Platform Binding (e.g. GLFW)
+
+// Implemented features:
+//  [X] Renderer: User texture binding. Use 'WGPUTextureView' as ImTextureID. Read the FAQ about ImTextureID!
+//  [X] Renderer: Support for large meshes (64k+ vertices) with 16-bit indices.
+
+// You can copy and use unmodified imgui_impl_* files in your project. See examples/ folder for examples of using this.
+// If you are new to Dear ImGui, read documentation from the docs/ folder + read the top of imgui.cpp.
+// Read online: https://github.com/ocornut/imgui/tree/master/docs
+
+#pragma once
+#include "imgui.h"      // IMGUI_IMPL_API
+
+// WebGPU
+#include <webgpu/webgpu.h>
+
+// cmd_list is the command list that the implementation will use to render imgui draw lists.
+IMGUI_IMPL_API bool ImGui_ImplWGPU_Init(WGPUDevice device, int num_frames_in_flight, WGPUTextureFormat rt_format);
+IMGUI_IMPL_API void ImGui_ImplWGPU_Shutdown();
+IMGUI_IMPL_API void ImGui_ImplWGPU_NewFrame();
+IMGUI_IMPL_API void ImGui_ImplWGPU_RenderDrawData(ImDrawData* draw_data, WGPURenderPassEncoder pass_encoder);
+
+// Use if you want to reset your rendering device without losing Dear ImGui state.
+IMGUI_IMPL_API void ImGui_ImplWGPU_InvalidateDeviceObjects();
+IMGUI_IMPL_API bool ImGui_ImplWGPU_CreateDeviceObjects();