st/mesa: when copying mipmap levels between textures, start at BaseLevel

Saves some work and avoids potential issue with inconsistant mipmap
level sizes.  As long as the mipmap levels from BaseLevel to MaxLevel
are consistant, we don't care about the other levels.

src/mesa/state_tracker/st_cb_texture.c

@@ -1900,7 +1900,7 @@ st_finalize_texture(GLcontext *ctx,
     */
    for (face = 0; face < nr_faces; face++) {
       GLuint level;
-      for (level = 0; level <= stObj->lastLevel; level++) {
+      for (level = stObj->base.BaseLevel; level <= stObj->lastLevel; level++) {
          struct st_texture_image *stImage =
             st_texture_image(stObj->base.Image[face][level]);