epicardium: implement FB API

q3krequested to merge
q3k/firmware:q3k/fb2 into master

This change:

  • introduces asynchronous screen updates and double buffering (by @msgctl and @schneider)
  • adds an epicardium call to retrieve the current buffer (epic_disp_frambuffer)
  • extends epic_disp_update to return -EAGAIN if the user tries to update/swap buffers when a swap is already occuring
Edited by q3k

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