QmlProfiler: Always reset the context before drawing on a canvas

The context is stateful and there could be any kind of junk left in it.
In particular, the W3C spec says that it should be set up with a default
clipping path on creation, but it doesn't say what should happen if the
canvas is resized. Thus, we can never be sure that the clipping path is
sane.

Task-number: QTBUG-38297
Change-Id: If04597234e71574573888ce14deac5cf50df4a3f
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>