The contents of TexManager._fontconfig determines whether the png for a given string/dpi/font combination is rendered from TeX or reused from the cache. Right now, there is no information of the custom latex preamble in _fontconfig, so a second string with a different latex preamble will reuse the png from the first call. Note that the tex and dvi files are generated as generating this depends on the _rc_cache which includes the latex preamble, so the expensive call to TeX is made but then the png is not used. This PR adds a hash of the latex preamble to TexManager._fontconfig so that a new png is read if preamble changes.

A minimal example which should produce two different images:

import matplotlib.pyplot as plt
plt.rcParams['text.usetex'] = True
plt.rcParams['font.family'] = 'serif'
for font,preamble in zip(['cm','times'], [' ',r'\usepackage{txfonts}']):
    plt.rcParams['text.latex.preamble'] = preamble
    f = plt.figure(figsize=(3,1))
    f.text(0.5,0.5,'This is some test text $e^{-i\pi}=-1$', ha='center',va='center')
    f.savefig('texmanager_test_{0}.png'.format(font))