shiny/driver/x11driver: fix the draw.Src operator.

This fixes example/tile, in that the output is now correct. That app now
flickers, but that will be fixed by either the TODO to recognize simple copies
or scales and the TODO to implement double-buffering.

Change-Id: I53ddd680243255dc116370d18490ff879ed9e5e2
Reviewed-on: https://go-review.googlesource.com/15674
Reviewed-by: David Crawshaw <crawshaw@golang.org>
2 files changed