TexManager.get_rgba fails

[astromancer]

Bug report

Bug summary

The example below is taken straight from the texmanager module docstring. The failure seems to be due to the function trying to stack numbers r, g, b with an array alpha without first converting them to arrays.

Code for reproduction

from matplotlib.texmanager import TexManager

tm = TexManager()
s = (r'\TeX\ is Number '
       r'$\displaystyle\sum_{n=1}^\infty\frac{-e^{i\pi}}{2^n}$!')
Z = tm.get_rgba(s, fontsize=12, dpi=80, rgb=(1, 0, 0))

Actual outcome

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-59-bb42fe32143e> in <module>
      5 s = (r'\TeX\ is Number '
      6        r'$\displaystyle\sum_{n=1}^\infty\frac{-e^{i\pi}}{2^n}$!')
----> 7 Z = tm.get_rgba(s, fontsize=12, dpi=80, rgb=(1, 0, 0))

/usr/local/lib/python3.6/dist-packages/matplotlib/texmanager.py in get_rgba(self, tex, fontsize, dpi, rgb)
    385         if Z is None:
    386             alpha = self.get_grey(tex, fontsize, dpi)
--> 387             Z = np.dstack([r, g, b, alpha])
    388             self.rgba_arrayd[key] = Z
    389 

/usr/local/lib/python3.6/dist-packages/numpy/lib/shape_base.py in dstack(tup)
    697     """
    698     _warn_for_nonsequence(tup)
--> 699     return _nx.concatenate([atleast_3d(_m) for _m in tup], 2)
    700 
    701 

ValueError: all the input array dimensions except for the concatenation axis must match exactly

Expected outcome

Expect an array Z to be retruned

Matplotlib version

• Operating system: Ubuntu 18.04.4 LTS
• Matplotlib version: 3.2.0rc2+1643.g122575cd5
• Python version: 3.6.9