alpha does not work in geom_raster like it does in geom_tile

[cwickham]

df <- expand.grid(x = 1:5, y = 1:5)
df$z <- rnorm(df$x)
qplot(x, y, data=df, geom = "tile", fill = z)
# works as expected
qplot(x, y, data=df, geom = "tile", alpha = z)

# same as geom_tile
qplot(x, y, data=df, geom = "raster", fill = z)
# UNEXPECTED not the same as geom tile
qplot(x, y, data=df, geom = "raster", alpha  = z)