Use different images as dummy images + rename them

The dummy images are really just intended as a fallback option and should
not be the target of texture packs.

README.txt

@@ -19,12 +19,12 @@ http://creativecommons.org/licenses/by-sa/3.0/
 
 Authors of media files
 -----------------------
-celeron55:
-  player.png
-  player_back.png
-
 MirceaKitsune (WTFPL):
   character.x
 
 Jordach (CC BY-SA 3.0):
   character.png
+
+Wuzzy (CC BY-SA 3.0)
+  playermodel_dummy_front.png (derivate work of character.png)
+  playermodel_dummy_back.png (derivate work of character.png)

player.lua

@@ -112,7 +112,7 @@ function default.player_set_model(player, model_name)
 		default.player_set_animation(player, "stand")
 	else
 		player:set_properties({
-			textures = { "player.png", "player_back.png", },
+			textures = { "playermodel_dummy_front.png", "playermodel_dummy_back.png", },
 			visual = "upright_sprite",
 		})
 	end