Previously QgsSvgCache would often try to fetch remote images
using a network request on the main thread, by calling
processEvents repeatedly until the request was complete.
This caused lots of bugs, since the main thread processEvents
would proceed with all kinds of stuff assuming that the
svg fetch operation was complete, leading to frequent crashes
and deadlocks and making remote svg use impossible (it's
likely that the SVG cache remote fetching code was written
in the pre-multi-threaded rendering era).
There's no way to fix this with async svg fetching - we
HAVE to remove the processEvents call, and a QEventLoop
won't help either (since the method may be called on the
main thread). Accordingly the only solution is to
fetch the requested svg in the background, and return
a temporary "downloading" svg for use in the meantime.
We use a QgsNetworkContentFetcherTask to do this, so it's
nicely integrated with task manager.
A request task is fired up when a remote svg is requested
for the first time, with the temporary downloading svg
returned for use by the caller asynchronously. QgsSvgCache
then emits the remoteSvgFetched signal when a previously
requested remote SVG has been successfully fetched,
triggering a map canvas redraw with the correct SVG
graphic.
Fixes#18504
