shiny/driver/windriver: Buffer & dummy Texture

Add Buffer implementation wrapping image.RGBA, and a dummy
implementation of a Texture. This allows to run shiny/example/basic on
Windows without early failure. This makes it easier to visually verify
implementation of further windriver methods in future.

Also, rename windriver types to screenImpl and windowImpl to match
naming convention used in x11driver.

Change-Id: I30b464e58ac3e973fd98344d221505980de7d5bd
Reviewed-on: https://go-review.googlesource.com/15686
Reviewed-by: Nigel Tao <nigeltao@golang.org>
4 files changed