Keep tile top-left corner fixed, don't center it

Makes it easier to see what happens when the parameters are tweaked.

Change-Id: Ia69c0de74ccf3fcc8a5095a7cc62c6b4185cf9d6
Make the use of tileWidth/Height in the call to PaintTile() optional

It seems to work better to use contextWidth/Height here, too,
converted to logical units. The code apparently will never render more
than what has been passed earlier to SetOutputSizePixel() anyway. Or
something.

Change-Id: I69ff4f3c9a61391882e5d530140903786bb4a16b