GraphicsContext for Qt: add popTransparencyLayerInternal method.

It is harmful to just call endPlatformTransparencyLayer inside
GraphicsContextQt.cpp,because the three calling sequences:

 1. beginPlatformTransparenceyLayer
    endPlatformTransparencyLayer

 2. pushTransparencyLayerInternal

 3. savePlatformState
    restorePlatformState

may interleave, which makes unexpected result.

Task-number: QTBUG-39577
Change-Id: I676a85cd5210e0f944cf6b522dd21deec06f03bc
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>