Add Android Vulkan backend (#2711)

Co-authored-by: Bart Louwers <bart.louwers@gmail.com>

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

@@ -20,8 +20,6 @@ public:
     PremultipliedImage readStillImage() override;
     RendererBackend* getRendererBackend() override;
 
-    void swap();
-
     class Impl {
     public:
         virtual ~Impl() = default;

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

@@ -24,6 +24,11 @@ public:
 
     void createPlatformSurface() override {}
     void bind() override {}
+
+    void swap() override {
+        SurfaceRenderableResource::swap();
+        static_cast<Context&>(backend.getContext()).waitFrame();
+    }
 };
 
 HeadlessBackend::HeadlessBackend(const Size size_,
@@ -69,10 +74,6 @@ gfx::Renderable& HeadlessBackend::getDefaultRenderable() {
     return *this;
 }
 
-void HeadlessBackend::swap() {
-    static_cast<Context&>(*context).waitFrame();
-}
-
 PremultipliedImage HeadlessBackend::readStillImage() {
     auto& contextImpl = static_cast<Context&>(*context);
     auto& resourceImpl = static_cast<HeadlessRenderableResource&>(*resource);