Don't destroy placeholder texture references until the end of a frame.

[jespertheend]

Right now the placeholder textures of a material are destroyed right after the new ones have been created for that material.
But this makes it possible for there to be a brief moment where there are no references for that color, even though right after that another material could potentially create a new reference with the same color.

Ideally all the placeholder texture references of each material are collected while a frame is being rendered, and at the very end of the render call the reference is removed.