Swap chains, part 1 (#87)

Adds the swap chain interfaces to the API without changing the behavior
of anything else. This includes the C APIs for applications to provide
swap chain implementations. Also adds stub implementations on every
backend.

src/backend/opengl/SwapChainGL.h

@@ -0,0 +1,42 @@
+// Copyright 2017 The NXT Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef BACKEND_OPENGL_SWAPCHAINGL_H_
+#define BACKEND_OPENGL_SWAPCHAINGL_H_
+
+#include "backend/SwapChain.h"
+
+#include "glad/glad.h"
+
+namespace backend {
+namespace opengl {
+
+    class Device;
+
+    class SwapChain : public SwapChainBase {
+        public:
+            SwapChain(SwapChainBuilder* builder);
+            ~SwapChain();
+
+        protected:
+            TextureBase* GetNextTextureImpl(TextureBuilder* builder) override;
+
+        private:
+            GLuint nativeTexture = 0;
+    };
+
+}
+}
+
+#endif // BACKEND_OPENGL_SWAPCHAINGL_H_