shiny/driver/x11driver: have textureImpl.draw use an opaque mask.

This make the output match gldriver when op == draw.Src even if the
texture itself isn't fully opaque.

Change-Id: Ib2aba62e68da71f32d876def1e1d672c3403d441
Reviewed-on: https://go-review.googlesource.com/23190
Reviewed-by: David Crawshaw <crawshaw@golang.org>
2 files changed