Expose Metal texture from headless backend (#4267)

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

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <mbgl/gfx/headless_backend.hpp>
+#include <mbgl/mtl/mtl_fwd.hpp>
 #include <mbgl/mtl/renderer_backend.hpp>
 #include <memory>
 #include <functional>
@@ -17,12 +18,14 @@ public:
     void updateAssumedState() override;
     gfx::Renderable& getDefaultRenderable() override;
     PremultipliedImage readStillImage() override;
+    MTL::Texture* getMetalTexture();
     RendererBackend* getRendererBackend() override;
     SwapBehaviour getSwapBehaviour();
 
 private:
     void activate() override;
     void deactivate() override;
+    void ensureResource();
 
 private:
     bool active = false;

platform/default/src/mbgl/mtl/headless_backend.cpp

@@ -2,6 +2,7 @@
 #include <mbgl/mtl/renderable_resource.hpp>
 #include <mbgl/mtl/offscreen_texture.hpp>
 #include <mbgl/mtl/context.hpp>
+#include <mbgl/mtl/texture2d.hpp>
 #include <mbgl/gfx/backend_scope.hpp>
 
 #include <Metal/Metal.hpp>
@@ -29,6 +30,10 @@ public:
 
     PremultipliedImage readStillImage() { return offscreenTexture->readStillImage(); }
 
+    MTL::Texture* getMetalTexture() {
+        return static_cast<Texture2D*>(offscreenTexture->getTexture().get())->getMetalTexture();
+    }
+
     const RendererBackend& getBackend() const override { return context.getBackend(); }
 
     const MTLCommandBufferPtr& getCommandBuffer() const override {
@@ -72,10 +77,14 @@ void HeadlessBackend::deactivate() {
 }
 
 gfx::Renderable& HeadlessBackend::getDefaultRenderable() {
+    ensureResource();
+    return *this;
+}
+
+void HeadlessBackend::ensureResource() {
     if (!resource) {
         resource = std::make_unique<HeadlessRenderableResource>(*this, static_cast<mtl::Context&>(getContext()), size);
     }
-    return *this;
 }
 
 void HeadlessBackend::updateAssumedState() {
@@ -86,6 +95,11 @@ PremultipliedImage HeadlessBackend::readStillImage() {
     return getResource<HeadlessRenderableResource>().readStillImage();
 }
 
+MTL::Texture* HeadlessBackend::getMetalTexture() {
+    ensureResource();
+    return getResource<HeadlessRenderableResource>().getMetalTexture();
+}
+
 RendererBackend* HeadlessBackend::getRendererBackend() {
     return this;
 }