rr doesn't need or use checkpoint/restore functionality. it's insufficient because rr already needs to catch and emulate all syscalls made by a process, and unnecessary because once you're doing that it's (relatively) easy to also emulate getpid. it also requires CAP_SYS_ADMIN or CAP_CHECKPOINT_RESTORE.

"a matching context" means having exactly the same handles for all objects (since application memory will have references to them) and with modern-enough OpenGL even means having the same (virtual) memory addresses for all GPU-side buffers as well as the same addresses for any host-side mappings. This is probably not something you could build on top of OpenGL or even on top of the existing kernel-side drivers.

Even if have a perfect wrapper that achieves all that you now have to save all application-supplied texture and shader data in the original form even where that memory could normally be freed after translating it to GPU-specific formats.

But "restricting yourself to a shared subset of OpenGL" alone already makes this unviable for anything demanding as it also means restricting yourself to the lowest common denominator for all limits including VRAM size, maximum texture sizes, essentially guaranteeing your solution to run worse on both systems.