Fix crash on windows viewer when starting in vulkan.

setRequestedDisplayParams is called when setting up viewer before we've
inited the Window. On windows this tries to attach the Window for a given
backend. However, we haven't set fBackend yet. Later on we will directly
call attach.

Change-Id: I4bd6586478f2b040e5913314c4e47e92fc893a60
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/344756
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>

tools/sk_app/Window.h

@@ -52,6 +52,7 @@ public:
     virtual bool scaleContentToFit() const { return false; }
 
     enum BackendType {
+        kUnknown_BackendType,
 #ifdef SK_GL
         kNativeGL_BackendType,
 #endif

tools/sk_app/android/Window_android.cpp

@@ -66,6 +66,8 @@ void Window_android::initDisplay(ANativeWindow* window) {
                     window_context_factory::MakeVulkanForAndroid(window, fRequestedDisplayParams);
             break;
 #endif
+        case kUnknown_BackendType:
+            SkUNREACHABLE;
     }
     this->onBackendCreated();
 }

tools/sk_app/ios/Window_ios.mm

@@ -100,6 +100,8 @@ bool Window_ios::attach(BackendType attachType) {
         case kRaster_BackendType:
             fWindowContext = MakeRasterForIOS(info, fRequestedDisplayParams);
             break;
+        case kUnknown_BackendType:
+            SkUNREACHABLE;
     }
     this->onBackendCreated();
 

tools/sk_app/mac/Window_mac.mm

@@ -148,6 +148,8 @@ bool Window_mac::attach(BackendType attachType) {
         case kRaster_BackendType:
             fWindowContext = MakeRasterForMac(info, fRequestedDisplayParams);
             break;
+        case kUnknown_BackendType:
+            SkUNREACHABLE;
     }
     this->onBackendCreated();
 

tools/sk_app/unix/Window_unix.cpp

@@ -418,6 +418,8 @@ bool Window_unix::attach(BackendType attachType) {
             fWindowContext =
                     window_context_factory::MakeRasterForXlib(winInfo, fRequestedDisplayParams);
             break;
+        case kUnknown_BackendType:
+            SkUNREACHABLE;
     }
     this->onBackendCreated();
 

tools/sk_app/win/Window_win.cpp

@@ -383,6 +383,8 @@ bool Window_win::attach(BackendType attachType) {
                 window_context_factory::MakeD3D12ForWin(fHWnd, fRequestedDisplayParams);
             break;
 #endif
+        case kUnknown_BackendType:
+            SkUNREACHABLE;
     }
     this->onBackendCreated();
 
@@ -403,7 +405,9 @@ void Window_win::setRequestedDisplayParams(const DisplayParams& params, bool all
         fWindowContext = nullptr;
         this->closeWindow();
         this->init(fHInstance);
-        this->attach(fBackend);
+        if (fBackend != kUnknown_BackendType) {
+            this->attach(fBackend);
+        }
     }
 
     INHERITED::setRequestedDisplayParams(params, allowReattach);

tools/sk_app/win/Window_win.h

@@ -35,7 +35,7 @@ private:
 
     HINSTANCE fHInstance;
     HWND      fHWnd;
-    BackendType fBackend;
+    BackendType fBackend = kUnknown_BackendType;
 
     using INHERITED = Window;
 };