svga: reset vertex buffer offset in svga_release_user_upl_buffers()

This function releases the buffer that contains user-space vertex data.
The buffer_offset field points into that buffer.  So reset the
buffer_offset to zero when we release the buffer so that subsequent
draws don't inadvertantly get a bad offset.

Fixes error messages / failed assertions (in the draw module's bounds/size
checking code) when running piglit's polygon-mode test.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>
(cherry picked from commit 04341e51ce)

src/gallium/drivers/svga/svga_pipe_draw.c

@@ -201,6 +201,17 @@ svga_release_user_upl_buffers(struct svga_context *svga)
       if (vb->buffer && svga_buffer_is_user_buffer(vb->buffer)) {
          struct svga_buffer *buffer = svga_buffer(vb->buffer);
 
+         /* The buffer_offset is relative to the uploaded buffer.
+          * Since we're discarding that buffer we need to reset this offset
+          * so it's not inadvertantly applied to a subsequent draw.
+          *
+          * XXX a root problem here is that the svga->curr.vb[] information
+          * is getting set both by gallium API calls and by code in
+          * svga_upload_user_buffers().  We should instead have two copies
+          * of the vertex buffer information and choose between as needed.
+          */
+         vb->buffer_offset = 0;
+
          buffer->uploaded.start = ~0;
          buffer->uploaded.end = 0;
          if (buffer->uploaded.buffer)