2/3 wined3d: add a helper function for drawing a textured quad

[Henri Verbeet]

On 17 March 2010 14:53, Roderick Colenbrander <thunderbird2k at gmail.com> wrote:
> This patch introduces a helper function for drawing a textured quad. A
> future patch will also make use of it for the quad drawing part of
> 'offscreen surface -> render target' in BltOverride (actually that
> part will become two functions later on: one for color keying to be
> called from Blt/BltFast and one without any color keying)
>
The idea of making this a separate function is probably ok, but that
function has more GL state dependencies than just locking and context
activation, and also invalidates some GL states. Ultimately drawing a
quad is something that should be internal to a blitter implementation.
A shader based blitter for example wouldn't have to invalidate lots of
fixed function state because it simply replaces those parts of the GL
pipeline. A FBO blit based implementation doesn't have to draw a quad
at all. Perhaps this is an acceptable step in that direction, but it's
probably a good idea to think about what the blitter interface should
look like first.

Also, const correctness.