[cogl] Don't allow calling cogl_set_source_texture with an INVALID_HANDLE
[cogl] Don't allow calling cogl_set_source_texture with an INVALID_HANDLE

Although the underlying materials should allow layers with INVALID_HANDLES
it shouldn't be necissary to expose that via cogl_set_source_texture() and
it's easier to resolve a warning/crash here than odd artefacts/crashes later
in the pipeline.

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