Do not set the mainframe graphicslayer to being opaque when it isn't

If the frameview has a transparent background we need to tell the
GraphicsLayer that it is not opaque otherwise the TextureMapperGL might
choose not to use blend.

Task-number: QTBUG-40063
Change-Id: If129305fed8620cd3c28cec5357625d8892ad063
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>