Add OpenGL triangle integration test with GLSL shaders

- Add tests/RHI/OpenGL/integration/triangle/ with main.cpp, CMakeLists.txt
- Add GLSL shaders (triangle.vert, triangle.frag) for vertex/fragment shading
- Use OpenGLVertexArray, OpenGLBuffer, OpenGLShader, OpenGLPipelineState
- Disable depth test to allow 2D triangle rendering
- GT.ppm reused from D3D12 triangle test (0% diff)

tests/RHI/OpenGL/integration/CMakeLists.txt

@@ -7,3 +7,4 @@ find_package(Python3 REQUIRED)
 enable_testing()
 
 add_subdirectory(minimal)
+add_subdirectory(triangle)