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/triangle/Res/Shader/triangle.frag

@@ -0,0 +1,8 @@
+#version 460
+
+in vec4 vColor;
+out vec4 fragColor;
+
+void main() {
+    fragColor = vColor;
+}

tests/RHI/OpenGL/integration/triangle/Res/Shader/triangle.vert

@@ -0,0 +1,11 @@
+#version 460
+
+layout(location = 0) in vec4 aPosition;
+layout(location = 1) in vec4 aColor;
+
+out vec4 vColor;
+
+void main() {
+    gl_Position = aPosition;
+    vColor = aColor;
+}