Fixed warning with AMD drivers

Occured when deleting a shader created by a deleted context, but since
shaders are shareable, this should not be an issue (OpenGL 4.2: D.1.1)
Added std::hash for String, made use of unordered_map
OpenGL's VAOs are now used the right way
Fixed MD5Anim loaders not being unregistred
Fixed (?) potential bug with Shader::SendTexture