add backend and update_state kwargs to show

[bjarthur]

as discussed show is the way to "save" a figure to an IOBuffer. but if the figure was made with say WGLMakie and you want to save as say PDF then it's a bit cumbersome because you have to CairoMakie.activate! and then WGLMakie.activate! after showing. this PR make it more concise by adding a backend kwarg to show, just like save has one already. so one can simply show(...; backend=CairoMakie) instead of activating and deactivating it.

Type of change

Delete options that do not apply:

• New feature (non-breaking change which adds functionality)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added unit tests for new algorithms, conversion methods, etc.

[bjarthur]

do any other show methods defined by Makie need to have a backend kwarg added as well?

also, i noticed that show is not tested at all. should i bother to add one?

[SimonDanisch]

Thank you, seems reasonable!

should i bother to add one?

Yeah why not :)

[bjarthur]

all i can think to test is for the presence of a backend kwarg to show.

ideally i'd like to test that specifying a different backend works, but that would require putting the test in one of the backend repos and adding a dependency to one of the other backends. you don't want to go that far, do you?

[bjarthur]

ready for review

[bjarthur]

the failed test on WGLMakie for julia 1.6 is a memory leak. is that related to this PR?

[bjarthur]

just added another kwarg to make reseting the axis limits optional when show'ing.

[EDIT: let me know if you'd prefer this to be in a separate PR]