Add WebGPU ImTextureID binding.

Image-Loading-and-Displaying-Examples.md

@@ -82,6 +82,7 @@ DirectX9:  ImTextureID = LPDIRECT3DTEXTURE9           (see ImGui_ImplDX9_RenderD
 DirectX11: ImTextureID = ID3D11ShaderResourceView*    (see ImGui_ImplDX11_RenderDrawData()    in imgui_impl_dx11.cpp)
 DirectX12: ImTextureID = D3D12_GPU_DESCRIPTOR_HANDLE  (see ImGui_ImplDX12_RenderDrawData()    in imgui_impl_dx12.cpp)
 Vulkan:    ImTextureID = VkDescriptorSet              (see ImGui_ImplVulkan_RenderDrawData()  in imgui_impl_vulkan.cpp)
+WebGPU:    ImTextureID = WGPUTextureView              (see ImGui_ImplWGPU_RenderDrawData()    in imgui_impl_wgpu.cpp)
 ```
 For example, in the OpenGL example binding we store raw OpenGL texture identifier (GLuint) inside ImTextureID. Whereas in the DirectX11 example binding we store a pointer to ID3D11ShaderResourceView inside ImTextureID, which is a higher-level structure tying together both the texture and information about its format and how to read it.
 - If you have a custom engine built over e.g. OpenGL, instead of passing GLuint around you may decide to use a high-level data type to carry information about the texture as well as how to display it (shaders, etc.). The decision of what to use as ImTextureID can always be made better knowing how your codebase is designed. If your engine has high-level data types for "textures" and "material" then you may want to use them. If you are starting with OpenGL or DirectX or Vulkan and haven't built much of a rendering engine over them, keeping the default ImTextureID representation suggested by the example bindings is probably the best choice. (Advanced users may also decide to keep a low-level type in ImTextureID, and use ImDrawList callback and pass information to their renderer)