Improve heart/bubble textures, add _gone textures

mods/default/README.txt

@@ -127,8 +127,6 @@ BlockMen (CC BY-SA 3.0):
   default_book.png
   default_paper.png
   default_stick.png
-  bubble.png
-  heart.png
   gui_*.png
 
 Glass breaking sounds (CC BY 3.0):
@@ -176,3 +174,11 @@ https://www.freesound.org/people/sonictechtonic/sounds/241872/
 Metal sounds:
   default_metal_footstep.ogg - Ottomaani138 - CC0
   - https://www.freesound.org/people/Ottomaani138/sounds/232692/
+
+An0n3m0us (CC BY-SA 3.0):
+  heart.png -- Derived from a texture by KevDoy (CC BY-SA 3.0)
+  bubble.png -- Derived from a texture by BlockMen (CC BY-SA 3.0)
+
+Wuzzy
+  heart_gone.png -- Derived from heart.png (CC BY-SA 3.0)
+  bubble_gone.png -- Derived from bubble.png (CC BY-SA 3.0)