loader: refactor layer activation function to handle more general inputs

Renamed structure types as well.

TODO: verify vk-layer-generate.py

v2: fix LoaderEntrypointsSubcommand (olv)
diff --git a/vulkan.py b/vulkan.py
index 07f431b..220b4a6 100755
--- a/vulkan.py
+++ b/vulkan.py
@@ -1024,11 +1024,11 @@
     print("core =", str(ext))
 
     print("")
-    print("typedef struct _VK_LAYER_DISPATCH_TABLE")
+    print("typedef struct VkLayerDispatchTable_")
     print("{")
     for proto in ext.protos:
         print("    vk%sType %s;" % (proto.name, proto.name))
-    print("} VK_LAYER_DISPATCH_TABLE;")
+    print("} VkLayerDispatchTable;")
 
 if __name__ == "__main__":
     parse_vk_h("include/vulkan.h")