Enable backend switching for Windows Viewer

tools/viewer/sk_app/win/GLWindowContext_win.cpp

 
 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "WindowContextFactory_win.h"
 #include <GL/gl.h>
@@ skipping 78 matching lines @@
         RECT rect;
         GetClientRect(fHWND, &rect);
         fWidth = rect.right - rect.left;
         fHeight = rect.bottom - rect.top;
         glViewport(0, 0, fWidth, fHeight);
     }
 }
 
 
 void GLWindowContext_win::onDestroyContext() {
-    wglMakeCurrent(wglGetCurrentDC(), NULL);
+    HDC dc = GetDC(fHWND);

[bsalomon, 2016/07/27 20:26:38]

Should this be paired with a ReleaseDC? Or do we really need to have a
wglMakeCurrent before wglDeleteContext?

From the docs it sounds like it just can't be current on another thread. If it's
current on the calling thread then it becomes not-current in wglDeleteContext.

[jvanverth1, 2016/07/27 20:41:50]

Deleted. I added this in to try and figure out why OpenGL wouldn't shut down
enough to let Vulkan create a swapchain. It doesn't need to be there now.

+    wglMakeCurrent(dc, NULL);
     wglDeleteContext(fHGLRC);
     fHGLRC = NULL;
 }
 
 
 void GLWindowContext_win::onSwapBuffers() {
     HDC dc = GetDC((HWND)fHWND);
     SwapBuffers(dc);
     ReleaseDC((HWND)fHWND, dc);
 }
 
 
 }  // anonymous namespace
 
 namespace sk_app {
 namespace window_context_factory {
 
 WindowContext* NewGLForWin(HWND wnd, const DisplayParams& params) {
     GLWindowContext_win* ctx = new GLWindowContext_win(wnd, params);
     if (!ctx->isValid()) {
         delete ctx;
         return nullptr;
     }
     return ctx;
 }
 
 }  // namespace window_context_factory
 }  // namespace sk_app