This reverts commit 3c47a3beb5.
Of course I remembered just after pushing the patch why we hadn't done
this before :-) If you look in the glsl spec:
http://www.khronos.org/registry/gles/specs/2.0/es_full_spec_2.0.24.pdf
Section 3.7.10 Texture Completeness and Non-Power-Of-Two Textures
you can see GLES 2.0 doesn't support mipmaps for npot textures.
There is possibly some way we could support this in Cogl but at least
it's not as simple as or-ing in the feature flag, sadly.
56daae9d7b