pbkit: Expose graphics context registration

[abaire]

These are particularly useful for 2d image blitting since pbkit doesn't set up the beta factors for blended blits.

Example usage: https://github.com/abaire/nxdk_pgraph_tests/blob/c2cb783bd63bf93dfdcba786bf9ce6c81d650114/src/tests/image_blit_tests.cpp#L161

[Ryzee119]

Thanks for providing the example code. I had a need for such things making my SDL3 port 👍