Images for emotes, badges, etc. are not lazily loaded as intended

[dnsge]

Checklist

• I'm reporting a problem with Chatterino
• I've verified that I'm running the most recent nightly build or stable release
• I've looked for my problem on the wiki
• I've searched the issues and pull requests for similar looking reports

Describe your issue

As @pajlada hinted at in #3915 (comment), lazy loading for images that aren't visible (e.g. in a different split tab) is broken.

Currently, images are supposed to be accessed by the pixmapOrLoad method in Image:

chatterino2/src/messages/Image.cpp

Lines 334 to 341 in c204332

	boost::optional<QPixmap> Image::pixmapOrLoad() const
	{
	assertInGuiThread();
	this->load();
	return this->frames_->current();
	}

This lazy loading works as long as no other code calls Image::load. However, in ImageSet, we call load manually:

chatterino2/src/messages/ImageSet.cpp

Lines 87 to 92 in c204332

	const ImagePtr &ImageSet::getImageOrLoaded(float scale) const
	{
	auto &&result = getImagePriv(*this, scale);
	// get best image based on scale
	result->load();

The intent behind calling load is that we will attempt to load the highest resolution of the image but fall back to any already loaded resolutions if they exist (see rest of method body).

This wouldn't be a problem if getImageOrLoaded was called in the rendering logic which only runs for the visible splits. However, this method is actually called in the message layout logic, which runs for all splits as messages are added:

chatterino2/src/messages/MessageElement.cpp

Lines 179 to 189 in c204332

	void EmoteElement::addToContainer(MessageLayoutContainer &container,
	MessageElementFlags flags)
	{
	if (flags.hasAny(this->getFlags()))
	{
	if (flags.has(MessageElementFlag::EmoteImages))
	{
	auto image =
	this->emote_->images.getImageOrLoaded(container.getScale());
	if (image->isEmpty())
	return;

I wanted to detail this problem and my investigation into it here so we can find a good solution for properly doing lazy loading. I couldn't immediately think of a solution that uses already-loaded images while queueing up higher resolution images.

This problem reduces the effectiveness of #3915 too, and fixing this problem should also lead to a reduction in memory usage by images.

OS and Chatterino Version

Chatterino 7.3.5 DEBUG (commit 2234670) built with Qt 5.15.2 Running on macOS 12.3, kernel: 21.4.0