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/include/nxt/nxt_wsi.h

@@ -0,0 +1,64 @@
+//* 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 NXT_WSI_H
+#define NXT_WSI_H
+
+#include <nxt/nxt.h>
+
+// Error message (or nullptr if there was no error)
+typedef const char* nxtSwapChainError;
+
+typedef struct {
+    /// Backend-specific texture id/name/pointer
+    void* texture = nullptr;
+} nxtSwapChainNextTexture;
+
+typedef struct {
+    /// Initialize the swap chain implementation.
+    ///   (*wsiContext) is one of nxtWSIContext{D3D12,Metal,GL}
+    void (*Init)(void* userData, void* wsiContext);
+
+    /// Destroy the swap chain implementation.
+    void (*Destroy)(void* userData);
+
+    /// Configure/reconfigure the swap chain.
+    nxtSwapChainError (*Configure)(void* userData, nxtTextureFormat format, uint32_t width, uint32_t height);
+
+    /// Acquire the next texture from the swap chain.
+    nxtSwapChainError (*GetNextTexture)(void* userData, nxtSwapChainNextTexture* nextTexture);
+
+    /// Present the last acquired texture to the screen.
+    nxtSwapChainError (*Present)(void* userData);
+
+    /// Each function is called with userData as its first argument.
+    void* userData = nullptr;
+} nxtSwapChainImplementation;
+
+#ifdef NXT_ENABLE_BACKEND_D3D12
+typedef struct {
+} nxtWSIContextD3D12;
+#endif
+
+#ifdef NXT_ENABLE_BACKEND_METAL
+typedef struct {
+} nxtWSIContextMetal;
+#endif
+
+#ifdef NXT_ENABLE_BACKEND_OPENGL
+typedef struct {
+} nxtWSIContextGL;
+#endif
+
+#endif // NXT_WSI_H