Add warning when raster data doesn't fall on an even grid

[thomasp85]

Fixes #2932

This adds a warning when geom_raster() can't truthfully represent the provided data. To avoid recompilation the call to resolution() has been changed to direct calculation.

@batpigandme can you have a look at the wording of the warning?

[clauswilke]

On first glance the code looks good to me but since the visual tests fail maybe there are some unanticipated side effects? The one difference I see between resolution() and your new code is that resolution() uses unique(as.numeric(x)) where you use unique(x).

[clauswilke]

And also please make sure things behave in a reasonable way when the data vector contains only a single unique value. Not sure if that's the case right now or not. At a minimum, the warning in that case seems confusing.

[yutannihilation]

I think this assumption is too strong. For example:

unique(diff(1:10 / 10))
#> [1] 0.1 0.1 0.1 0.1

^{Created on 2019-04-24 by the reprex package (v0.2.1)}

We need a version of unique() with tolerance.

[lock]

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