copy dirty lines to screen, add select() timeout & min time between draw() calls.

copy dirty lines to screen, add select() timeout & min time between draw() calls.

* add a timeout value (SELECT_TIMEOUT) of 20ms in the select() call
* wait at least 20ms (DRAW_TIMEOUT) between draw() calls
* only copy dirty lines from the buffer to the screen

what draw() does:
* clears dirty lines in the buffer
* draws the longest same-attributes string of each
  dirty line to the buffer with multiple xdraws() call
* copies the current dirty line from buffer to the screen with a single
  xcopy() call

this changeset makes st run ~10x faster.