render: Fix undefined shift in xRenderColorToCard32

Delightfully this is undefined even though CARD16 is an "unsigned" type,
because it's an unsigned short, which C promotes to signed int because
everything is stupid.
3 jobs for master in 6 minutes and 44 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed #759029
debian-testing

00:00:30

 
  Build And Test
passed #759030
autotools-build-and-test

00:06:14

passed #759031
meson-build-and-test

00:01:53