Make RenderTextureEmulation configurable

[James McKenzie]

Ove Kaaven wrote:
> This makes it possible to work around bug 10080 without having to patch
> the source code.
>   
This is a hack to allow a work around to function.  Much better to
actually fix the problem permanently. 

Also, it is preferred to preface your patch with the affected area like
this:

[winex11.drv]  Patch to fix problem in bug #xxxxxx.

James McKenzie