pbkit: Enable flip stall; expose ramin, print_char, depth buffer internals

[antangelo]

This PR upstreams the pbkit changes required by the pgraph test suite.

They are all combined into one PR, but can be split it if preferred. CC: @abaire

[JayFoxRox]

Commit titles don't follow our usual style (<prefix>: <Verb> [...] without any punctuation at the end)