Use tile coordinates for the DRAW_ONLY_TILE functionality

CATiledLayer does not guarantee that tiles are drawn in the same order
each time so using a "tile number" for DRAW_ONLY_TILE was not
perfect. Use tile coordinates instead when wanting to restrict to
showing just one tile.

Change-Id: I23f4a3ecaf47cd3392d2d950bd279260b3a7b9f4