support SpatRaster

[rhijmans]

With these changes you can run the example in help(slideView) with SpatRasters

slideView(terra::rast(img2000), terra::rast(img2013), label1 = "before", label2 = "after")

I would suggest replacing the Raster* methods with a single method

setMethod("slideView", signature(img1 = "Raster", img2 = "Raster"),
          function(img1,  img2, ...) {
                 slideView(terra::rast(img1), terra::rast(img2), ...)
          }
)

and explain that some arguments are only relevant when there is 1 layer

[tim-salabim]

Thanks @rhijmans! A method switch from raster to terra is a good idea.