offscreen: Check the texture exists before setting
offscreen: Check the texture exists before setting

When creating a cogl texture, check the creation was successful before
setting it on the ClutterTexture. This fixes a possible critical warning
when the MxOffscreen is set to an invalid size for a texture.

pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx