Container spec indicates that background color is written
in BGRA byte order, but glClearColor() parameters are RGBA.
Anyway the checkerboard is displayed right after glClear()
calls so it replaces background color.
In response to webp-discuss/TkLHALGaHaM
Change-Id: Ief5435fadfd6a422b881a9dc240b5e8dc6546e19