optimised texture loading for ofxAssimpModelLoader.
previously a texture was loaded for every mesh without checking if the texture already existed.
this approach saves on any unnecessary texture loading.
textures now holds a vector of ofxAssimpTexture, this fixes a small bug that crept in.
Merge pull request #1974 from julapy/assimp_image_load_optimisation

ofxAssimpModelLoader - optimised texture loading.