Backends: SDLGPU3: Added sdl_gpu backend. (#8163, #7998, #7988)

+Squashed: Optimized shader source code encoding by ocornut (#8163, #7998, #7988)
(squashed to avoid storing both in git history, 130 KB->62 KB)

backends/imgui_impl_sdlgpu3.h

@@ -0,0 +1,46 @@
+// dear imgui: Renderer Backend for SDL_Gpu
+// This needs to be used along with the SDL3 Platform Backend
+
+// Implemented features:
+//  [X] Renderer: User texture binding. Use simply cast a reference to your SDL_GPUTextureSamplerBinding to ImTextureID.
+//  [X] Renderer: Large meshes support (64k+ vertices) with 16-bit indices.
+
+// The aim of imgui_impl_sdlgpu3.h/.cpp is to be usable in your engine without any modification.
+// IF YOU FEEL YOU NEED TO MAKE ANY CHANGE TO THIS CODE, please share them and your feedback at https://github.com/ocornut/imgui/
+
+// You can use unmodified imgui_impl_* files in your project. See examples/ folder for examples of using this.
+// Prefer including the entire imgui/ repository into your project (either as a copy or as a submodule), and only build the backends you need.
+// Learn about Dear ImGui:
+// - FAQ                  https://dearimgui.com/faq
+// - Getting Started      https://dearimgui.com/getting-started
+// - Documentation        https://dearimgui.com/docs (same as your local docs/ folder).
+// - Introduction, links and more at the top of imgui.cpp
+
+// Important note to the reader who wish to integrate imgui_impl_sdlgpu3.cpp/.h in their own engine/app.
+// - Unline other backends, the user must call the function Imgui_ImplSDLGPU_PrepareDrawData BEFORE issuing a SDL_GPURenderPass containing ImGui_ImplSDLGPU_RenderDrawData.
+//   Calling the function is MANDATORY, otherwise the ImGui will not upload neither the vertex nor the index buffer for the GPU. See imgui_impl_sdlgpu3.cpp for more info.
+
+#pragma once
+#include "imgui.h"      // IMGUI_IMPL_API
+#ifndef IMGUI_DISABLE
+#include <SDL3/SDL_gpu.h>
+
+// Initialization data, for ImGui_ImplSDLGPU_Init()
+// - Remember to set ColorTargetFormat to the correct format. If you're rendering to the swapchain, call SDL_GetGPUSwapchainTextureFormat to query the right value
+struct ImGui_ImplSDLGPU_InitInfo
+{
+    SDL_GPUDevice*       GpuDevice          = nullptr;
+    SDL_GPUTextureFormat ColorTargetFormat  = SDL_GPU_TEXTUREFORMAT_INVALID;
+    SDL_GPUSampleCount   MSAASamples        = SDL_GPU_SAMPLECOUNT_1;
+};
+
+// Follow "Getting Started" link and check examples/ folder to learn about using backends!
+IMGUI_IMPL_API bool             ImGui_ImplSDLGPU_Init(ImGui_ImplSDLGPU_InitInfo* info);
+IMGUI_IMPL_API void             ImGui_ImplSDLGPU_Shutdown();
+IMGUI_IMPL_API void             ImGui_ImplSDLGPU_NewFrame();
+IMGUI_IMPL_API void             Imgui_ImplSDLGPU_PrepareDrawData(ImDrawData* draw_data, SDL_GPUCommandBuffer* command_buffer);
+IMGUI_IMPL_API void             ImGui_ImplSDLGPU_RenderDrawData(ImDrawData* draw_data, SDL_GPUCommandBuffer* command_buffer, SDL_GPURenderPass* render_pass, SDL_GPUGraphicsPipeline* pipeline = nullptr);
+IMGUI_IMPL_API bool             ImGui_ImplSDLGPU_CreateFontsTexture();
+IMGUI_IMPL_API void             ImGui_ImplSDLGPU_DestroyFontsTexture();
+
+#endif // #ifndef IMGUI_DISABLE