Text.{get,set}_usetex: manually enable/disable TeX

On some machines with slow TeX rendering, it might be useful to be able to disable usetex for specific `matplotlib.text.Text` objects.

My specific use-case is running a live data Animation on a slow, old machine, with TeX rendering for all labels and ticks, but disabling it for the time-stamp that updates every second - meaning dvipng isn't getting called continuously. Might be useful in other cases.