Electron: Off-screen rendering trusts GPU-supplied geometry over shared-memory size

Electron is a framework for writing cross-platform desktop applications using JavaScript, HTML and CSS. Prior to 39.8.10, 40.9.0, 41.2.1, and 42.0.0-beta.3, offscreen rendering frame data received from the GPU process was not fully validated by the main process. A compromised GPU process could cause the main process to read out-of-bounds memory while producing paint event images, disclosing memory or crashing the app. This issue is fixed in 39.8.10, 40.9.0, 41.2.1, and 42.0.0-beta.3.