Remove QSGContext and QSGTexture from QDeclarativePixmapData

In order to cleanly support multiple windows we need to not
have the QSGContext reference in QDeclarativeEngine and we
need to be able to have one copy of the texture for each
QSGContext that is not sharing when we have multiple windows.

This also makes it easier to the release graphical resources from
a running application, which is another feature that we want
to eventually implement.

This patch does remove an adaptation feature which is the
decodeToTexture, which seemed like a good idea at the time
but in hindsight has not been used for anything.

Edit: resolved conflict in qquickimage.cpp
Edit: updated QDeclarativePixmap testcase
Edit: updated QQuickImage testcase
Edit: Rebased on top of QDeclarativePixmapCacheChanges

Change-Id: Ifc61dd8158d3f841437d029b6031a91196145517
Reviewed-by: Martin Jones <martin.jones@nokia.com>