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 ubsan-fixes in 4 minutes and 32 seconds (queued for 10 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #750477

00:00:18

 
  Build And Test
passed autotools-build-and-test #750478

00:04:08

passed meson-build-and-test #750479

00:01:45