Add a warning message when no usable Cogl pipeline backend is found
Add a warning message when no usable Cogl pipeline backend is found

If none of the pipeline backends can handle the state then it now
displays a warning so the developer has a chance to work out what's
going on.

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