Introduce QOpenGLTexture::target()

To extract the target of a given texture object. Somehow this
accessor was missing.

Change-Id: Ie43366bed3627a20204600e68e426b55abf37af6
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>