build: Fix potentially uninitialized VS2015 warning

This fixes the a few occurences in demos, the loader and a test.
It also adds the warning to the always-on list so it will prevent
future regressions.

Fixes #1587.

Change-Id: I26f69e977b57749a3ab4ddb548ada95384131edc
diff --git a/tests/layer_validation_tests.cpp b/tests/layer_validation_tests.cpp
index 9ef889d..88bc089 100644
--- a/tests/layer_validation_tests.cpp
+++ b/tests/layer_validation_tests.cpp
@@ -1355,7 +1355,7 @@
     TEST_DESCRIPTION("Create images with sparse residency with unsupported types");
 
     // Determine which device feature are available
-    VkPhysicalDeviceFeatures available_features;
+    VkPhysicalDeviceFeatures available_features = {};
     ASSERT_NO_FATAL_FAILURE(GetPhysicalDeviceFeatures(&available_features));
 
     // Mask out device features we don't want
@@ -1421,7 +1421,7 @@
     TEST_DESCRIPTION("Create images with sparse residency with unsupported tiling or sample counts");
 
     // Determine which device feature are available
-    VkPhysicalDeviceFeatures available_features;
+    VkPhysicalDeviceFeatures available_features = {};
     ASSERT_NO_FATAL_FAILURE(GetPhysicalDeviceFeatures(&available_features));
 
     // These tests all require that the device support sparse residency for 2D images
@@ -16069,7 +16069,7 @@
 
     ASSERT_NO_FATAL_FAILURE(InitState());
 
-    VkPhysicalDeviceFeatures device_features;
+    VkPhysicalDeviceFeatures device_features = {};
     ASSERT_NO_FATAL_FAILURE(GetPhysicalDeviceFeatures(&device_features));
     VkFormat compressed_format = VK_FORMAT_UNDEFINED;
     if (device_features.textureCompressionBC) {
@@ -16547,7 +16547,7 @@
     }
 
     // Test compressed formats, if supported
-    VkPhysicalDeviceFeatures device_features;
+    VkPhysicalDeviceFeatures device_features = {};
     ASSERT_NO_FATAL_FAILURE(GetPhysicalDeviceFeatures(&device_features));
     if (!(device_features.textureCompressionBC || device_features.textureCompressionETC2 ||
           device_features.textureCompressionASTC_LDR)) {