svga: use the draw-module's sprite stage depending on FS inputs

svga: use the draw-module's sprite stage depending on FS inputs

If we're drawing sprites and the fragment shader needs both auto-
generated texcoords and user-defined varying vars we need to use
this fallback path.
The reason is when we enable auto texcoord generation, it gets
enabled for all texcoord sets.  And that clobbers the user-defined
varying vars.

Reviewed-by: José Fonseca <jfonseca@vmware.com>