Add OpenGL support to Linux viewer

tools/viewer/Viewer.cpp

Index: tools/viewer/Viewer.cpp
diff --git a/tools/viewer/Viewer.cpp b/tools/viewer/Viewer.cpp
index c4b8b26dc51e3fda5a43a66962f5dba0a397de0d..b450f2192d8a6dc2d8fb5f9796564fef955ee2d6 100644
--- a/tools/viewer/Viewer.cpp
+++ b/tools/viewer/Viewer.cpp
@@ -35,7 +35,7 @@ static bool on_touch_handler(int owner, Window::InputState state, float x, float
     return viewer->onTouch(owner, state, x, y);
 }
 
-DEFINE_bool2(fullscreen, f, true, "Run fullscreen.");
+DEFINE_bool2(fullscreen, f, false, "Run fullscreen.");
 DEFINE_string(key, "", "Space-separated key/value pairs to add to JSON identifying this builder.");
 DEFINE_string2(match, m, nullptr,
                "[~][^]substring[$] [...] of bench name to run.\n"
@@ -47,6 +47,7 @@ DEFINE_string2(match, m, nullptr,
                "If a bench does not match any list entry,\n"
                "it is skipped unless some list entry starts with ~");
 DEFINE_string(skps, "skps", "Directory to read skps from.");
+DEFINE_bool(vulkan, true, "Run with Vulkan.");
 
 const char *kBackendTypeStrings[sk_app::Window::kBackendTypeCount] = {
     " [OpenGL]",
@@ -72,6 +73,9 @@ Viewer::Viewer(int argc, char** argv, void* platformData)
 
     SkCommandLineFlags::Parse(argc, argv);
 
+    fBackendType = FLAGS_vulkan ? sk_app::Window::kVulkan_BackendType
+                                : sk_app::Window::kNativeGL_BackendType;
+
     fWindow = Window::CreateNativeWindow(platformData);
     fWindow->attach(fBackendType, DisplayParams());
 
@@ -117,6 +121,7 @@ Viewer::Viewer(int argc, char** argv, void* platformData)
         this->changeZoomLevel(-1.f / 32.f);
         fWindow->inval();
     });
+#if 0  // this doesn't seem to work on any platform right now
 #ifndef SK_BUILD_FOR_ANDROID
     fCommands.addCommand('d', "Modes", "Change rendering backend", [this]() {
         fWindow->detach();
@@ -124,15 +129,17 @@ Viewer::Viewer(int argc, char** argv, void* platformData)
         if (sk_app::Window::kVulkan_BackendType == fBackendType) {
             fBackendType = sk_app::Window::kNativeGL_BackendType;
         } 
-        // TODO: get Vulkan -> OpenGL working without swapchain creation failure
+        // TODO: get Vulkan -> OpenGL working on Windows without swapchain creation failure
         //else if (sk_app::Window::kNativeGL_BackendType == fBackendType) {
         //    fBackendType = sk_app::Window::kVulkan_BackendType;
         //}
 
         fWindow->attach(fBackendType, DisplayParams());
         this->updateTitle();
+        fWindow->inval();
     });
 #endif
+#endif
 
     // set up slides
     this->initSlides();