Fix assert in BitmapTextureImageBuffer::applyFilters()

While WebCore should not give a BitmapTextureImageBuffer any filters
applyFilters() will still be called with an empty list of filters.

We should therefore not assert that applyFilters is never called by
that is only ever called with an empty list of filters to apply.

Change-Id: Ic24b735f1fc92e2a57c7ef068a8c7d712f0fc31f
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Fix assert in m_stack.isEmpty() in GraphicsContext::~GraphicsContext()

Save and restore is not balanced in GraphicsContext. This causes
the log error "ERROR void GraphicsContext::restore() stack is empty"
and assert in the GraphicsContext destructor.

The cause is a mismatch of GraphicsContexts being saved and restored by
TextureMapperImageBuffer beginClip and endClip. This patch fixes endClip
to restore the currentContext beginClip saved.

Change-Id: Ief0100dd26c7eded04734167e86712a81f1a7674
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>