RAM usage quake on upsizing the window

## I'm submitting a…
Bug Report

## Current Behavior
When I upsize (f.e., maximize) the LuaKit window, I get a RAM usage quake of `luakit` process: it grows to more than 1 GB for a while, than falls back to normal.

## Expected Behavior
No abrupt changes of RAM usage.

## Reproduction Instructions
This seems to be related to the large number of tabs or the large session again. I just tried to spawn 64 empty tabs in a clean session, and couldn't reproduce such a quake. However, the stable RAM usage of `luakit` process differs depending on the window size:
* quarter of screen: 105 MB;
* half of screen: 168 MB;
* full screen: 283 MB.

I have favicons and vertical tabs enabled, and a compositing WM (Compiz), if matters.

## Environment

Linux Distribution & Version: Debian 10.
Output of `luakit --version`: luakit 2.0.0
built with webkit 2.22.6 (installed version: 2.24.1)

Start by reproducing the resize behavior with luakit 2.0.0 on Debian 10, using the reported window sizes, tab counts, and session conditions. Compare memory usage during and after maximizing, including the WebKit version shown by `luakit --version`; done means the transient increase is explained and no longer exceeds the expected usage.

Written by the indexing model from the issue text.