Avoid sRGB usage when targeting an incapable FBO

Avoid sRGB usage when targeting an incapable FBO

Native text rendering results in very different output on OS X
with QQuickWidget/QOpenGLWidget. When targeting an FBO, enabling
sRGB is futile if the FBO was not created with the correct format.
In this case we need to disable the usage of sRGB.

Task-number: QTBUG-42861
Change-Id: I887482e70be2cbfba40d6758546128d03b36def1
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>