Add OpenGL test framework with logging and screenshot comparison

- Unified resolution to 1280x720 (same as D3D12)
- Added XCEngine Logger integration for debug output
- Added screenshot capture at frame 30 (glReadPixels -> PPM)
- Added run.bat test script
- Added compare_ppm.py for screenshot comparison
- Added GT.ppm reference image
- Added README.md documentation
- Updated CMakeLists.txt to link XCEngine library

Test: OpenGL rendering test passed (screenshot comparison 100% match)

tests/OpenGL/run.bat

@@ -0,0 +1,7 @@
+@echo off
+cd /d "%~dp0..\..\..\..\build\tests\OpenGL\Debug"
+if exist "OpenGL_engine_log.txt" del "OpenGL_engine_log.txt"
+if exist "screenshot.ppm" del "screenshot.ppm"
+XCRender.exe
+python "%~dp0compare_ppm.py" "screenshot.ppm" "GT.ppm" 5
+pause