scene: Introduce render graph texture type
This is the prerequisite for introducing render nodes and layer textures.
The KrkTexture class provides an API similar to the QSGTexture's API.
The KrkTexture is effectively split in two parts - the generic render graph texture and the platform-specific texture, e.g. OpenGL texture or QImage. The split is necessary to allow implementing atlas textures.
The decoration item puts decoration borders in a texture atlas. This is not feasible currently as every decoration part will need a separate geometry node and we don't have paint nodes yet. It'll be added later.
Note that a scene graph and a render graph are different. A scene graph contains items such as surface items. A render graph specifies how the items must be rendered. For now, there is no render graph, but it will contain things such as texture render nodes, transform nodes, etc.