retro_run - we need to escape first before we can get the next libretro
core with input data running. This is not needed when we do exec without
being in the main program loop. Anyway, one does not work for the other -
so we need both.
Moved private stuff to libretro_private.h.
Dropped use of retro_variable (redundant and wrong type).
Didn't understand difference between EXEC_LOAD and EXEC_RELOAD at all.
Only one was used anyways ...
