Bad texture files would cause the background decoder thread to never
finish and would generate textures with 0x0 dimensions, which eventually
triggers GL_INVALID_VALUE.
To recap, gltfio texture loading has two phases: the first phase is
synchronous and uses stbi_info to gather texture info. The second phase
is asynchronous and uses stbi_load to do the actual decoding.
During the first phase, we now check the return code from stbi_info
rather than assuming that it worked. :)
Fixes#3344
5c666c254e