Reduce memory usage of QQuickPath for simple paths.

For a single segment line path we can get correct results by
interpolating across just two points, so there no need to allocate
the comparatively large point cache required for more complex paths.

This also correct a normally small interpolation error caused by
including the terminal point in the number of intervals.

Change-Id: I564c3c012b6822831251276951603fca8544a356
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>