Add support for native rasters to annotation_raster() #3388

[kent37]

Small change to allow passing a nativeRaster object to annotation_raster(). This is a significant performance improvement for large rasters.

I didn't include an example because I don't see a source for a native raster in {ggplot2}. I could use the R logo from {png} but that package is not currently used by ggplot2. If you think an example is called for, please advise.

[clauswilke]

Could you merge in the master branch so the unit tests don't fail?

[kent37]

@clauswilke I'm not sure I understand what you want me to merge. My branch is based on the tip of the master branch; do you want me to merge it into master myself?

[clauswilke]

Ah, sorry, we had a bunch of PRs last week that were based off of a version of master that was failing unit tests, but that's not your case. Looks like your checks are passing, except one random issue on travis.

[kent37]

Yes, the Travis issue doesn't seem to be related to my change, it is failing to install R 3.2. Let me know if there is anything else you want me to do.

[clauswilke]

No, I think it's fine, thanks!