VulkanDevice: Request indenpendentBlend.

This is a core NXT feature but wasn't enabled, causing the Vulkan
validation layers to produce an error.

src/backend/vulkan/VulkanBackend.cpp

@@ -471,6 +471,9 @@ namespace backend { namespace vulkan {
             usedKnobs->swapchain = true;
         }
 
+        // Always require independentBlend because it is a core NXT feature,
+        usedKnobs->features.independentBlend = VK_TRUE;
+
         // Find a universal queue family
         {
             constexpr uint32_t kUniversalFlags =