macOS: Default to Vulkan for rendering (project-slippi#306)

* Change macOS to default to Vulkan in 10.14+.

This mirrors a change in mainline Dolphin, so should be relatively safe
to do. Slippi's MoltenVK setup is a bit older than modern Dolphin's, but
most users still see a jump in performance in comparison to Apple's
broken OpenGL API.

High Sierra remains defaulted to OpenGL, as Metal on that platform lacks
critical pieces that Dolphin requires.

Source/Core/VideoCommon/VideoBackendBase.cpp

@@ -85,8 +85,26 @@ void VideoBackendBase::PopulateList()
 	// disable OGL video Backend while is merged from master
 	g_available_video_backends.push_back(std::make_unique<OGL::VideoBackend>());
 
-    if(PlatformSupportsVulkan()) {
-	    g_available_video_backends.push_back(std::make_unique<Vulkan::VideoBackend>());
+	// on macOS, we want to push users to use Vulkan on 10.14+ (Mojave onwards). OpenGL has been 
+	// long deprecated by Apple there and is a known stumbling block for performance for new players.
+	//
+	// That said, we still support High Sierra, which can't use Metal (it will load, but lacks certain critical pieces).
+	//
+	// This mirrors a recent (2021) change in mainline Dolphin, so should be relatively safe to do here as well. All
+	// we're doing is shoving Vulkan to the front if it's macOS 10.14 or later, so it loads first.
+	if(PlatformSupportsVulkan()) {
+#ifdef __APPLE__
+		if (__builtin_available(macOS 10.14, *)) {
+			g_available_video_backends.emplace(
+				g_available_video_backends.begin(),
+				std::make_unique<Vulkan::VideoBackend>()
+			);
+		} 
+		else
+#endif
+        	{
+	        	g_available_video_backends.push_back(std::make_unique<Vulkan::VideoBackend>());
+        	}
     }
 
 	// Disable software video backend as is currently not working