scene: Generate quads in item local coordinate space

Window quads need to be in some coordinate space. Since we want items to
be used not only for rendering windows, window-local coordinates do not
suffice.

This change makes scene items generate quads in the item-local coordinate
space.

The model matrix is used to map the quads to the global screen coordinate
system from the item coordinate space.

Since the quads are in the item local coordinate space, the mvp matrix
needs to be updated on every draw call. Given the render data, tracking
the last mvp matrix won't result in less glUniform calls. If this indeed
becomes a serious performance bottleneck, we can explore the possibility
of dumping mvp matrices in a UBO, which have been introduced in OpenGL
3.1.