Fix readPixels to work with custom framebuffer.

ReadPixels always read from renderFbo if context had antialiasing
specified.

Also added buffer initialization to zero so pixels that fall outside
the framebuffer contain correct value.

Change-Id: I58b9fda18bc56a64346f83ccda575c758e9eaac5
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>