Add WebGPU Backend w/ Dawn Support (#3838)

platform/default/include/mbgl/webgpu/headless_backend.hpp

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <mbgl/gfx/headless_backend.hpp>
+#include <mbgl/webgpu/renderer_backend.hpp>
+#include <memory>
+
+namespace mbgl {
+namespace webgpu {
+
+class HeadlessBackend final : public webgpu::RendererBackend, public gfx::HeadlessBackend {
+public:
+    HeadlessBackend(Size = {256, 256},
+                    SwapBehaviour = SwapBehaviour::NoFlush,
+                    gfx::ContextMode = gfx::ContextMode::Unique);
+    ~HeadlessBackend() override;
+
+    gfx::Renderable& getDefaultRenderable() override;
+    PremultipliedImage readStillImage() override;
+    RendererBackend* getRendererBackend() override;
+
+    // Override WebGPU-specific methods
+    void* getCurrentTextureView() override;
+    void* getDepthStencilView() override;
+    mbgl::Size getFramebufferSize() const override;
+
+private:
+    void activate() override;
+    void deactivate() override;
+    void createOffscreenTextures();
+
+private:
+    class Impl;
+    std::unique_ptr<Impl> impl;
+    bool active = false;
+};
+
+} // namespace webgpu
+} // namespace mbgl