Restart pgf's latex instance after bad latex inputs.

[anntzer]

... and also move the warning about pgf-to-stream not supporting raster
images to when actually outputting a raster image (otherwise the warning
is spurious, and affects e.g. test_tex_restart_after_error).

backport of #15969. closes https://discourse.matplotlib.org/t/user-warning-steamed-pgf-code-does-not-support-raster-graphics/20934.

(this was not detected previously because we didn't cover the pgf+savefig("tight") combo)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[timhoffm]

Modulo @QuLogic ´s comment.

[anntzer]

Agreed it's a typo. Away from my computer, if either of you want to force push it for me please do.

[QuLogic]

Well, that didn't work, but how come it didn't fail on master?

[QuLogic]

OK, on master, the latex call uses encoding='utf-8', so what's read is already a string. I've gone and reverted the change to that line instead since what's there on v3.2.x is a bytearray.