texture: add NGLI_TEXTURE_TYPE_NB
texture: add NGLI_TEXTURE_TYPE_NB

This will be useful to perform checks before accessing a map between
NGLI_TEXTURE_TYPE_* and its backend implementation counterpart.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl