Implement sceGeGetStack(), fix sceGeGet*()

I'm not sure what games use these, but sceGeGetMtx() was not returning correct results, so it could help some game calling it.

I don't think any games use sceGeGetStack(), but it's useful for debugging and verifying the stack anyway.  This way we can test that offset/base are saved in the stack properly much more easily.

-[Unknown]