st: alternative scrollback using ring buffer and view offset

st: alternative scrollback using ring buffer and view offset

Implement scrollback as a fixed-size ring buffer and render history
by offsetting the view instead of copying screen contents.
Implement reflow of history and screen content on resize if it is needed.

Tradeoffs / differences:
  - Scrollback is disabled on the alternate screen
  - Simpler model than the existing scrollback patch set
  - Mouse wheel scrolling enabled by default
  - Shift + page up/down and shift + end/home work as well.
  - When using vim, mouse movement will no longer move the cursor.
  - There can be visual artifacts if width of the window is shrank to the
    size smaller than the shell promp.
  - Mouse selection is persistent even if it goes off screen but it get
    reset on resize.