[Feat] (Diligent): add real offscreen renderer with clear and triangle draw

- Add DiligentRenderer: creates an offscreen RGBA8 render target, compiles a
  GLSL vertex/pixel shader through Diligent's glslang path, creates a triangle
  vertex buffer and pipeline, and supports clear/draw/readback.
- BackendObject_Diligent now owns a DiligentRenderer after device creation.
- Extend local sanity test to clear green, draw a red triangle, and verify
  center is red and corner stays green.
- All Diligent local tests pass on Turnip Adreno 750.

CMakeLists.txt

@@ -416,6 +416,7 @@ if(MOBILEGL_ENABLE_DILIGENT)
     list(APPEND SOURCE_FILES
         MobileGL/MG_Backend/Diligent/BackendObject_Diligent.cpp
         MobileGL/MG_Backend/Diligent/DiligentVulkan.cpp
+        MobileGL/MG_Backend/Diligent/Renderer/DiligentRenderer.cpp
     )
 endif()