FramebufferGL: add readPixels workarounds Implements workarounds for: - The pack state making rows overlap in memory, which causes crashes on some drivers. - The driver adding an extra last row padding when checking if the pixel pack buffer is large enough for the readPixels. BUG=angleproject:1512 Change-Id: I120ff58649bb523e8b01da6ef03d8fcadaf076b2 Reviewed-on: https://chromium-review.googlesource.com/388029 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>