ICNS plugin: allow loading images over the network without warnings

If you try this QML code

import QtQuick 2.1

Rectangle {
    width: 360
    height: 360

    AnimatedImage {
        anchors.centerIn: parent
        source: "http://libmng.com/MNGsuite/images/T01SCRL.mng"
    }
}

it works, but it displays the warning

QICNSHandler::canRead() called on a sequential device

That's coming from here

bool QICNSHandler::canRead(QIODevice *device)
{
    if (!device || !device->isReadable()) {
        qWarning("QICNSHandler::canRead() called without a readable device");
        return false;
    }

    if (device->isSequential()) {
        qWarning("QICNSHandler::canRead() called on a sequential device");
        return false;
    }

    return device->peek(4) == QByteArrayLiteral("icns");
}

If the goal is just to read the first 4 bytes, it should be possible to do that even on a sequential device, and find some way to cache them for later. Perhaps a caching wrapper stream of some sort. Then the warning would not be necessary.

It's interesting that the ICNS plugin got invoked for MNG images though, isn't it? I guess it just checks to see if it canRead() and then tries another plugin. But you could put an icns file on a web server if you like, so this could maybe come up somewhere legitimately...

But if a lot of plugins will need to read some bytes in their canRead() functions, it's inefficient to query the first few bytes from the server repeatedly. Maybe we need another solution then.