layers: simplify initLayerTable()

Generate xgl_dispatch_table_helper.h with

 $ ./xgl-generate.py dispatch-table-ops layer

and make use of layer_initialize_dispatch_table() in initLayerTable().

There should be no functional difference with this change.
diff --git a/.gitignore b/.gitignore
index ce64764..09c4b1c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,6 +14,7 @@
 tests/xgl_render_tests
 tests/xglbase
 tests/xglinfo
+layers/xgl_dispatch_table_helper.h
 layers/xgl_enum_string_helper.h
 layers/xgl_struct_string_helper.h
 layers/xgl_struct_wrappers.cpp