Negative indices should count backwards from the last explicit
row/column number. If the resulting line number would appear before
the first explicit row/column, implicit grid lines should be added
before the first explicit row/column.

Previously, a touch on a component outside the Viewport would interrupt
and cancel a scroll gesture inside the Viewport.

Now, the Viewport will respond to all drag events from the input source
that started the drag, allowing the Viewport to be scrolled with one
input source while adjusting other controls with another input source.

The FontsDemo is useful for testing this behaviour, as it has two
Viewports on a single screen, along with some other controls.

See BREAKING-CHANGES.txt for more details.

This fixes warnings that are emitted when building with the `-analyze`
flag enabled.