Fix MSAA stripes (#9247)

This only works when shaders are enabled.
The centroid varying avoids that the textures (which repeat themselves out of bounds) are sampled out of bounds in MSAA.
If MSAA (called FSAA in minetest) is disabled, the centroid keyword does nothing.

builtin/settingtypes.txt

@@ -517,8 +517,8 @@ texture_min_size (Minimum texture size) int 64
 #    This algorithm smooths out the 3D viewport while keeping the image sharp,
 #    but it doesn't affect the insides of textures
 #    (which is especially noticeable with transparent textures).
-#    This option is experimental and might cause visible spaces between blocks
-#    when set above 0.
+#    Visible spaces appear between nodes when shaders are disabled.
+#    If set to 0, MSAA is disabled.
 #    A restart is required after changing this option.
 fsaa (FSAA) enum 0 0,1,2,4,8,16