The fundamental change of approach in this patch is that now the temporary
texture is per-compositable.
Originally, the temporary texture was per-TextureHost. That was too many
temporary textures. With Nical's work in bug 875211, that switched to having a
temporary texture per compositor only. That's what turned out to be too few.
Now we have one per compositable which is fewer than one per TextureHost,
because e.g. a ContentHost, which is a single Compositable, may have 2
TextureHosts to implement double-buffering.
