qt: introduce `QSGTextureView` and use it for zero-copy sub-texturing instead of layering

• I introduce a new view class for QSGTexture.
• This class allows representing a sub-part of an already existing QSGTexture, without modifying the source texture.
• Currently, we use layering to apply effect on the source. This is in addition to the initial layering for the source item, which makes it a texture provider.
• Although the layering for the effect area is not as expensive as the initial layer (because initial layer already makes a texture, second layer simply re-renders the texture in an offscreen buffer providing a new dynamic texture), it is still not necessary.
• Ideally this should be sent to upstream Qt, but that would take a long time. So we can merge it here.

Request review @chub.