At least on windows, trying to create a context with 0 alpha bits and
8 bits for other colors creates a context with 8 alpha bits.

Change-Id: I23659e4d47a0adee38249d85034a2bac41b84d1f
Task-number: QTBUG-45588
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>