mesa: simplify parameters to GetTexImage() driver hook

The target, level and texObj can be obtained through the texImage
parameter.  We could make similar changes for the TexImage() hooks too.

Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/src/mesa/main/debug.c b/src/mesa/main/debug.c
index 0a393e5..6e695d1 100644
--- a/src/mesa/main/debug.c
+++ b/src/mesa/main/debug.c
@@ -268,9 +268,7 @@
       store = ctx->Pack; /* save */
       ctx->Pack = ctx->DefaultPacking;
 
-      ctx->Driver.GetTexImage(ctx, texObj->Target, level,
-                              GL_RGBA, GL_UNSIGNED_BYTE,
-                              buffer, texObj, img);
+      ctx->Driver.GetTexImage(ctx, GL_RGBA, GL_UNSIGNED_BYTE, buffer, img);
 
       /* make filename */
       _mesa_snprintf(s, sizeof(s), "/tmp/tex%u.l%u.f%u.ppm", texObj->Name, level, face);