Visual glitches on web view

[dmorais-ledger]

When opening the Speculos web view in a browser (http://0.0.0.0:5000/) with BoilerPlate App (built for LNX/LNS+), some visual glitches may appear on the displayed screenshot when navigating in application's screens.

(happen on current master branch: 1af74ce)

Steps to reproduce:

• buid Boilerplate app for LNX or LNS+ (using API_LEVEL_5 branch on the SDK)
• start Speculos without any parameter (except the path to the .elf file) => a Qt window will be displayed.
• open a browser (tested with both Chrome & FireFox) and use that URL: http://0.0.0.0:5000/
• navigate through the application's screens using either Left/Right buttons on the browser, or Left/Right keys with Qt window.
  => from time to time, the displayed screenshot will contain part of previous application's screen with part of current application's screen (check screenshots below).