This speeds up the smooth rasterizer by avoiding a
conditional branches in the hot path. Namely:

- Define a fixed "null cell" which will be pointed
  to whenever the current cell is outside of the current
  target region. This avoids a "ras.cell != NULL"
  check in the FT_INTEGRATE() macro.

- Also use the null cell as a sentinel at the end of
  all ycells[] linked-lists, by setting its x coordinate
  to INT_MAX. This avoids a 'if (!cell)' check in
  gray_set_cell() as well.

- Slightly change the worker struct fields to perform
  a little less operations during rendering.

Example results (on a 2013 Corei5-3337U CPU)

  out/ftbench -p -s10 -t5 -bc /usr/share/fonts/truetype/droid/DroidSansFallbackFull.ttf

    Before: 5.472 us/op
    After:  5.275 us/op

  out/ftbench -p -s60 -t5 -bc /usr/share/fonts/truetype/droid/DroidSansFallbackFull.ttf

    Before: 17.988 us/op
    After:  17.389 us/op