st/mesa: fix pipe_get_transfer() call in fallback_copy_texsubimage()

Commit 1a339b6c71 caused us to take
a different path through the glCopyTexSubImage() code.  The
pipe_get_transfer() call neglected to pass the texture's level, face
and slice info.  So we were always transferring from the 0th mipmap
level even when the source renderbuffer was a non-zero mipmap level
in a texture.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=38649

NOTE: This is a candidate for the 7.10 branch.

src/mesa/state_tracker/st_cb_texture.c

@@ -1241,7 +1241,8 @@ fallback_copy_texsubimage(struct gl_context *ctx, GLenum target, GLint level,
 
    src_trans = pipe_get_transfer(pipe,
                                  strb->texture,
-                                 0, 0,
+                                 strb->rtt_level,
+                                 strb->rtt_face + strb->rtt_slice,
                                  PIPE_TRANSFER_READ,
                                  srcX, srcY,
                                  width, height);