Add frame rate control to desktop_opengl.cpp: support vsync and manual FPS modes

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This PR adds comprehensive frame rate control functionality to src/examples/desktop_opengl.cpp, enabling developers to switch between vertical sync and manual frame rate limiting for benchmarking and testing purposes.

Features Added

Frame Rate Modes

• VSync Mode (Default): Synchronizes rendering with display refresh rate for smooth, tear-free rendering
• Manual FPS Mode: Limits frame rate to a specific target (10-1000 FPS) using high-precision timing

Command Line Options

# Set manual frame rate and disable vsync
./desktop_opengl --fps 120

# Enable vertical sync (default behavior)
./desktop_opengl --vsync

# Combined with existing options
./desktop_opengl --fps 30 --mono -w 1920 -h 1080

Runtime Controls

• V key: Toggle between VSync and manual FPS modes
• + key: Increase target FPS (manual mode only)
• - key: Decrease target FPS (manual mode only)

Implementation Details

The implementation uses std::chrono::high_resolution_clock for microsecond-precision timing and std::this_thread::sleep_for() for accurate frame pacing. The frame rate control logic:

1. VSync Mode: Uses glfwSwapInterval(1) with no artificial limiting
2. Manual Mode: Calculates target frame time and sleeps to maintain precise FPS
3. Runtime Switching: Seamlessly toggles glfwSwapInterval() setting

Testing & Validation

Comprehensive testing shows excellent timing accuracy:

• 30 FPS target achieves 29.94 FPS (99.8% accuracy)
• 60 FPS target achieves 59.70 FPS (99.5% accuracy)
• VSync mode runs unlimited without constraints
• Smooth transitions between modes during runtime

Benefits

• Benchmarking: Test performance at different frame rates
• Power Efficiency: Lower FPS reduces battery usage on mobile devices
• Development: Debug timing-sensitive rendering issues
• Flexibility: Adapt to different display types and use cases

The implementation maintains full backward compatibility and adds minimal overhead while providing essential tools for performance analysis and testing.

Fixes #264.

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