Vulkan example: Fix validation layer warnings and errors and redeclare gl_PerVertex.

examples/vulkan_example/glsl_shader.vert

@@ -8,6 +8,10 @@ layout(push_constant) uniform uPushConstant{
     vec2 uTranslate;
 } pc;
 
+out gl_PerVertex{
+    vec4 gl_Position;
+};
+
 layout(location = 0) out struct{
     vec4 Color;
     vec2 UV;