Fix SoftwareRectangleNode creating QSGImageNode's without texture (take 2)

There are a couple of issues:

• if you call setFiltering() before setTexture(), the software renderer will crash
• if you leave a QSGImageNode without a texture, the software renderer will crash
• the cached texture obtained in SoftwareRectangleNode::setImage() can be destroyed before the preprocess() function is called

This change reverts the state of things prior to 4dc4e78d and focuses mainly on the texture provider code path. Unfortunately, it's not as clean as with unified code paths for static images and texture providers in 4dc4e78d.