EGE currently only supports Windows via GDI. This PR adds the foundation for cross-platform support by introducing a multi-backend rendering architecture with an opt-in OpenGL backend.

Changes

New INIT_OPENGL flag

• Added INIT_OPENGL = 0x200 to initmode_flag enum in both ege.h and ege.zh_CN.h

Backend abstraction layer (src/backend/)

• render_backend.h - Abstract interface for rendering backends
• render_backend.cpp - Backend factory and lifecycle management
• gdi_backend.h/.cpp - GDI wrapper (stubs with TODOs for integration)
• opengl_backend.h/.cpp - OpenGL 2.1 implementation using GLFW/GLAD

CMake configuration

• New EGE_ENABLE_OPENGL option (OFF by default)
• GLFW/GLAD detection with fallback to 3rdparty/ submodules

Demo and docs

• demo/graph_opengl_test.cpp - Usage example
• docs/CROSS_PLATFORM.md - Bilingual documentation

Usage

cmake .. -DEGE_ENABLE_OPENGL=ON

// Opt-in to OpenGL backend
ege::initgraph(640, 480, ege::INIT_OPENGL);

// Default GDI backend unchanged
ege::initgraph(640, 480, ege::INIT_DEFAULT);

Next steps

• Add GLFW/GLAD as submodules
• Wire backend into initgraph()
• Implement full GDI backend delegation

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