Returns a pixmap with its canvas expanded (a background-coloured border added).
ExpandCanvas(pm, x[, y])
pmis a pixmap
yare non-negative integers
ExpandCanvas(pm, x, y) returns
pm with its canvas expanded. Specifically,
x pixels of background colour are added to the left and right sides and
y pixels of background colour are added to the upper and lower sides, together increasing the width of the pixmap by
2⋅x and the height by
2⋅y and leaving the original pixmap centred in the result.
y defaults to