[cogl-material] Allow setting a layer with an invalid texture handle
[cogl-material] Allow setting a layer with an invalid texture handle

It was previously possible to create a material layer with no texture
by setting some property on it such as the matrix. However it was not
possible to get back to that state without removing the layer and
recreating it. It is useful to be able to remove the texture to free
resources without forgetting the state of the layer so we can put a
different texture in later.

gcampax/cogl,spatulasnout/cogl,djdeath/cogl,collects/cogl,djdeath/cogl-android,Distrotech/cogl,djdeath/cogl-android,gcampax/cogl,gcampax/cogl,djdeath/cogl,Distrotech/cogl,djdeath/cogl,collects/cogl,gcampax/cogl,spatulasnout/cogl,Distrotech/cogl,djdeath/cogl-android,Distrotech/cogl