Fixed blendTile()-related blurriness issue

We were setting drawer.updateAgain to the result of each blendTile(),
which meant it was keeping only the last result. Instead we should have
been only setting it to true if blendTile returned true, but never
setting it to false. Fixed.
Merge pull request #112 from openseadragon/blur

Fixed blendTile()-related blurriness issue (for #95)