[Pixman] [PATCH v11] Implement PIXMAN_FILTER_GOOD/BEST

[spitzak at gmail.com]

Changes form previous version:
- whitespace and formatting fixes
- removed "fixes" for IMPULSE.x at size < 1 or IMPULSE.IMPULSE, just let them
  produce unusable filters, since caller can avoid doing this.
- removed patch that deleted range checks
- simplification of the filter_width function
- fixed the comment to use "size" instead of "scale"