don't resample raster images when writing out to png
Merge remote-tracking branch 'origin/master' into noresample
update raster output to continue using old behaviour when !is.na(tikzRasterResolution)
fixed typo, missed a couple of simple cases in the rasterisation path
suppress warnings from Xlib/Windows png output when width/height < 20 pixels
am working on OS X, so didn't notice this before!
oops, quartz and X11 png files don't look the same (different gamma?)  the test builds run from Linux, so convert to an X11 PNG
fixing of issues pointed out by krlmlr and general tidy up
Merge remote-tracking branch 'yihui/master' into noresample
provide some documentation of the changes to tikzRasterResolution handling in docs
added entries to the NEWS.md file
Merge pull request #55 from smason/noresample

stop raster output from being resampled