egg-mkfont and egg-palettize should use png format by default, instead of rgb

pandatool/src/egg-mkfont/eggMakeFont.cxx

@@ -332,7 +332,7 @@ run() {
   }
   if (_output_glyph_pattern.empty()) {
     // Create a default texture filename pattern.
-    _output_glyph_pattern = get_output_filename().get_fullpath_wo_extension() + "%03d.rgb";
+    _output_glyph_pattern = get_output_filename().get_fullpath_wo_extension() + "%03d.png";
   }
   if (_output_palette_pattern.empty()) {
     // Create a default texture filename pattern.

pandatool/src/palettizer/palettizer.cxx

@@ -116,7 +116,7 @@ Palettizer() {
   _coverage_threshold = 2.5;
   _aggressively_clean_mapdir = true;
   _force_power_2 = true;
-  _color_type = PNMFileTypeRegistry::get_global_ptr()->get_type_from_extension("rgb");
+  _color_type = PNMFileTypeRegistry::get_global_ptr()->get_type_from_extension("png");
   _alpha_type = (PNMFileType *)NULL;
   _shadow_color_type = (PNMFileType *)NULL;
   _shadow_alpha_type = (PNMFileType *)NULL;