Fix Issue #416: Implement dynamic resource file location for testbed

[killerdevildog]

This commit addresses the hardcoded relative path issue that caused problems when different compilers/IDEs placed executables in different directories.

Key Changes:

• Added ResourceManager class for dynamic resource path discovery
• Searches up to 4 parent directories for testbed/shaders and testbed/meshes
• Supports RP3D_RESOURCE_PATH environment variable override
• Maintains backwards compatibility with fallback to relative paths
• Uses std::filesystem for cross-platform path handling

Files Added:

• testbed/common/ResourceManager.h - Header with dynamic path discovery API
• testbed/common/ResourceManager.cpp - Implementation with filesystem search
• dynamicresourcefilelocation.md - Comprehensive documentation

Files Modified:

• testbed/src/SceneDemo.cpp - Updated shader and mesh folder paths
• testbed/common/*.cpp - Updated all hardcoded mesh paths (Box, Sphere, etc.)
• testbed/scenes//.cpp - Updated scene-specific mesh paths
• testbed/CMakeLists.txt - Added ResourceManager to build system

Benefits:

• Cross-platform compatibility across different build environments
• No manual configuration required for developers
• Robust error handling with informative fallback warnings
• Performance optimized with path caching

Tested successfully with CMake build system - application now runs without file not found errors regardless of build directory structure.