EXT_direct_state_access: add support selectorless matrix stack functions
This MR is a followup of https://firstname.lastname@example.org/msg203342.html
It only implements the matrix related functions (glMatrix*), based on Chris Forbes / Timothy Arceri original implementations + some bugfixes.
I also opened a MR for piglit which contains tests for these functions (see piglit!38 (closed)).
I have more parts of EXT_dsa already implemented but I think it'll be easier to review in several small MR.