watermark: Place watermark in the bottom-right when both deltax and deltaty are 0

The default behavior is to replicate the watermark all over the screen but this can
be too intrusive. The repetition is controlled by deltax and deltay and if they are
both 0 the watermark will actually be drawn off-screen and thus not shown.

We take advantage of this to add a special behavior when they are both 0 that simply
places them in the bottom right. This patch doesn't take UI elements into account and
may actually overlay soft-buttons and other chrome.

Change-Id: I4cff1326b8aaecdbc471eaf48e95bc2865181366