Using grid_metrics with a raster instead of a resolution preserves the old values

[Jean-Romain]

When using a RasterLayer as layout in grid_metrics() the values of the reference are preserved:

library(lidR)
#> Le chargement a nécessité le package : raster
#> Le chargement a nécessité le package : sp

LASfile <- system.file("extdata", "Megaplot.laz", package="lidR")
ldr = readLAS(LASfile)
ldr10 = lasfilter(ldr, abs(ScanAngleRank) <= 5)

# Expected behavior
ref = lidR:::rOverlay(ldr, 10)
plot(grid_metrics(ldr10, mean(Z), ref))

# Invalid behavior
ref[] <- 1
plot(grid_metrics(ldr10, mean(Z), ref))

^{Created on 2020-01-29 by the reprex package (v0.3.0)}