Add pdsg (palladium spirv-generator)

- remove glslang from pd-system
- use spirv blobs instead of glsl 460
- Use Legacy shaders for OpenGL2 backend
- add devnotes

backends/include/pd_system/spirv-helper.hpp

@@ -1,10 +1,8 @@
 #pragma once
 
-#include <glslang/Public/ResourceLimits.h>
-#include <glslang/Public/ShaderLang.h>
-
 #include <pd/common.hpp>
 
+struct TBuiltInResource;
 namespace PD {
 class SpirvHelper {
  public: