Subtexture textures are not properly cleaned

[wouterlucas]

When there are subtextures that have 0 renderable owners, the subtexture is never freed.

Tested by:

• Spawn a subtexture node
• Listen for freed events on subtexture node
• Create noise textures until you hit the memory threshold
• Move subtexture node off screen

Results:
No freed event happens and texture remains in memory

Expected results:
Subtexture node would be freed up by the texture memory manager when off screen