rasterize traps that extend outside of the image bounds correctly

rasterize traps that extend outside of the image bounds correctly

Traps that extend outside of the image bounds are rasterized incorrectly
currently; the problem is a signed vs. unsigned comparison that got
introduced when a width parameter went from int -> uint.  This patch
puts it back to int (since it's stored as an int in the bits structure
to begin with), and also fixes a similar warning in the memset wrapper