Avoid rebuilding aggregate raytrace scene buffers every frame

[anirul]

Summary

• stop invalidating aggregate OpenGL raytrace scene buffers by frame time
• stop invalidating aggregate Vulkan raytrace scene buffers by elapsed time
• add buffer generations and no-op unchanged buffer copies so aggregate scene refresh only runs when raytrace source state actually changes

Root cause

Both backends were rebuilding aggregate raytrace scene triangles/BVH every frame even when the scene was static. The client-level regression showed up as a large FPS drop because acceleration structure work was happening continuously instead of only when source geometry or transforms changed.

Verification

• cmake --build --preset windows-client-debug in grpcMMO
• cmake --build C:\Users\frede\github\Frame\build\windows --config Debug --target FrameOpenGL FrameVulkan

Notes

Dynamic raytrace sources still rebuild the aggregate scene when their actual state changes. This PR only removes the unconditional per-frame rebuild behavior for unchanged scenes.