panel.levelplot.raster: fix for y being a factor

[aitap]

Steps to reproduce:

library(lattice)

x <- expand.grid(factor = factor(letters[1:3]), numeric = 1:4)
x$value <- runif(nrow(x))

levelplot(value ~ factor + numeric, x, useRaster = TRUE) # works well
levelplot(value ~ numeric + factor, x, useRaster = TRUE) # fails

The code fails due to a typo: the y.is.factor branch creates a variable named ux but later the code expects to work with uy.