Fix build for downstream VulkanSC uses

- video headers don't exist on this branch (and are not supported for Vulkan SC)
- loader interface files need to use vulkansc include instead of vulkan
- update documentation

include/vulkan/vk_icd.h

@@ -23,7 +23,12 @@
 #ifndef VKICD_H
 #define VKICD_H
 
+#if defined(VULKANSC)
+#include "vulkan_sc.h"
+#else
 #include "vulkan.h"
+#endif
+
 #include <stdbool.h>
 
 // Loader-ICD version negotiation API.  Versions add the following features:

include/vulkan/vk_layer.h

@@ -26,7 +26,12 @@
  */
 #pragma once
 
+#if defined(VULKANSC)
+#include "vulkan_sc.h"
+#else
 #include "vulkan.h"
+#endif
+
 #if defined(__GNUC__) && __GNUC__ >= 4
 #define VK_LAYER_EXPORT __attribute__((visibility("default")))
 #elif defined(__SUNPRO_C) && (__SUNPRO_C >= 0x590)