Composite against white, not the savefig.facecolor rc, in print_jpeg. - #15239

jpeg doesn't support alpha transparency, so we need to paste the image
against some solid background before saving it. Pasting against the
savefig.facecolor rc, as the code did before, is just wrong: it makes
e.g.

plot([1, 2])
rcParams["savefig.facecolor"] = "blue"
savefig("/tmp/foo.jpg", facecolor=(1, 0, 0, .5))

result in a purple background (blue + 0.5 red), instead of a red one (as
for png -- the facecolor kwarg should just override the rcParam).
Likewise the test_jpeg_alpha test was just wrong (indeed, trying to run
the same code but saving as png shows that transparent=True should
override the savefig.facecolor rcParam).

Instead, paste the image against a white background, which fixes the
issues (and is tested by the fixed test_jpeg_alpha).

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