Work around FBO corruption issue with some GPUs

Somehow the first couple of FBOs canvas creates can get corrupted
with some GPUs/drivers when completely unrelated Qt Quick items are
shown or modified on screen. Work around the issue by creating a couple
of dummy FBOs before creating our actual FBOs.

Change-Id: I5fb93535238dd69bd5d5fa51b1d09930830e41e0
Task-number: QTBUG-46101
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>