Display API change:
    - getTile -> startTile (no longer returns a tile)
    - setSubTile -> subTile (drop img/tile first parameter)
    - putTile -> finishTile (no longer takes img/tile paramter)

The Display tile logic uses canvas image data directly and
caches/reuses a 16x16 imageData tile (for other sizes, the tile is
create for each call). This gives a 30% speedup on Chrome
13 (and no significant change for Firefox 3.6/4.0).

Other:

- Remove rgbxImageFill and cmapImageFill routines.

- Simplify constructor tests and just error if createImageData is not
  supported by canvas instead of .

- Remove webkit canvas bug workaround that effects Chrome 7. Chrome
  7 usage share is now less than 0.5 percent and the workaround is
  ugly. Drop the function wrapping in the constructor and the canvas
  flush() routine.

- Remove support for getImageData (Opera 11+ now required)

Update browser support list:

    - Chrome 8+ (really any except 7)
    - Firefox 3.6+
    - Safari 4+
    - Opera 11+
    - IE9+
    - iOS 4.2+