Enable backend switching for Windows Viewer

tools/viewer/sk_app/win/GLWindowContext_win.cpp

Index: tools/viewer/sk_app/win/GLWindowContext_win.cpp
diff --git a/tools/viewer/sk_app/win/GLWindowContext_win.cpp b/tools/viewer/sk_app/win/GLWindowContext_win.cpp
index eca8829d3d747f25831e5a9c6d40cafda81827ca..0c1e944ce4e424a114edffb3e31934dfcfc98766 100644
--- a/tools/viewer/sk_app/win/GLWindowContext_win.cpp
+++ b/tools/viewer/sk_app/win/GLWindowContext_win.cpp
@@ -96,7 +96,8 @@ void GLWindowContext_win::onInitializeContext() {
 
 
 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;
 }