Fix AttributeError when saving figure with partial Latex elements as a postscript (issue 7741)

[ibnIrshad]

Fixes #7741

The PostScript backend relies on rcParams['text.usetex'] being set explicitly in order to load TeX correctly. So we set rcParams when the Axes arg usetex is set.

The fix can be verified by running this bug reproduction script (it will crash with an AttributeError unless this PR's patch is applied).

import matplotlib
import matplotlib.pyplot as plt
fig = plt.figure()
plt.text(0.5, 0.5, 'Here is some text, $math mode$', ha='center', va='center', usetex=True)
fig.savefig('test.ps')

[tacaswell]

Single artists should not change global state when changed.

This change would prevent having some text on the screen that use latex and some that does not.

[tacaswell]

The first half of this (grapping usetex from kwargs is good), but the other half of this is probably going to have to be in the ps backend.

[ibnIrshad]

@tacaswell I agree that it seems hacky to modify global state in a single artist...but I think it is appropriate here. The PS backend relies on the rcParams['text.usetex'] setting in order to initialize its latex texmanager, which is then used by all subsequent latex text renderings. This rcParams setting does not affect the rendering of every Text object (you can still have non-latex objects on the screen after setting rcParams to true), as it only controls the initialization of latex-specific managers. I think modifying the global state is appropriate in this case, because if a single artist requires latex, then the backend should initialize latex-specific stuff, even if it is not used for all the remaining text objects.

There is probably a better way to do this (by making the PS backend NOT rely on rcParams for initializing its latex-stuff, but that is a LOT more work).

[jklymak]

Thanks for the PR. This hasn't seen action for a long time so marking stale. Note that PRs sometimes fall through the cracks, so if this is important, please ping for more reviews.

[jklymak]

Closing due to no action.