mkspec macx-ios-clang: ensure SDK version is valid

Ensure the sdk is of recent enough version since:
1. we build Qt with the latest sdk version, so the app needs
    to do the same to avoid compatibility problems e.g when linking.
2. using a launch screen to support iphone6 depends on sdk 8
3. Apple requires apps that are pushed to appstore to use the
    latest version of the sdk.

Ideally we should store the sdk version used to build Qt, and
require that apps use the same version or newer. But this patch
will do until that is in place.

Change-Id: I18b06d09c1eda15122975b7169ca7a3372df6054
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Fix for current_fbo getting out of sync in QtOpenGL

When using QGLWidget in combination with QOpenGLFramebufferObject from
QtGui, instead of QGLFramebufferObject from QtOpenGL, the current_fbo
variable doesn't get updated when framebuffer object bindings change.

To ensure that the QGLWidget correctly releases the currently bound
framebuffer object when using a QPainter, we keep track of whether
QOpenGLFramebufferObject has modified the current FBO binding, and if
that's the case we need to read the OpenGL state directly instead of
relying on a cached value.

Change-Id: If7e0bd936e202cad07365b5ce641ee01d2251930
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Fixed QtGui's GL paint engine getting out of sync when using QtOpenGL

We need to reset the active_engine belonging to QOpenGLContext whenever
we make the QtOpenGL paint engine active, to give the OpenGL paint
engine in QtGui a chance to sync its state if we've used the QtOpenGL
paint engine inbetween.

Change-Id: I445ce2f99bfbacf55650c881c4fdf07f2ff85069
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>