QSvgIOHandler: trigger lazy load in metadata getters

imageCount(), nextImageDelay(), and currentImageNumber() returned
uninitialized values when called before any other getter that triggers
the document load. Add the missing d->load() calls so that the results
do not depend on prior call history.

Also fix imageCount() to return 1 for static (non-animated) SVGs,
and fix currentImageNumber() to return the last read frame number.

Amends 6b0aba5544c4e2c9015a691a7ad669b3d6c94cfe.

Fixes: QTBUG-148685
Pick-to: 6.12
Change-Id: I510d5fc5b99daa2ac7caba6ccdd1c992d173ed74
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>