Make sure all themes have visual tests

[karawoo]

Adds visual tests for theme_linedraw() and theme_test().

[hadley]

You don't need to test theme_test() (and in fact you definitely shouldn't test it), because it's the theme used for most tests and should never change (if it does will have much breakage in the tests).

Per @baptiste's suggestion, can you please also test all with base_size = 33 (under the assumption if it looks good at 100% and 300% it's probably good in between)

[karawoo]

Ah ok, I reverted the theme_test() test.

I'm a little confused about what the new tests should be testing. As far as I can tell the base_size argument only affects text elements (and a few margins). The changes in 2173 don't alter that, so I'm not sure if this will address @baptiste's concerns about the line widths. Should line/rect elements scale with base_size as well?

[hadley]

Oh good point. Maybe we should have a base line size too?

[karawoo]

I can add that if you want. Would this be additional argument to the theme functions, or would it be better to set the base line size within the theme according to the base font size, and then let people override that with + theme(line = element_line(size = 1)) or whatever? Either way this seems like it deserves its own PR. I think the AppVeyor build failed on this one for reasons unrelated to the changes I made, but I don't see a way to force start a new build the way I can with Travis...

[hadley]

I think it would work in the same way as base_size - i.e. line = element_line(size = base_line_size). I guess it would default to base_size / 22? I agree that this should be a different PR, and you can go ahead and merge this one.

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