Refactor many definitions out of topics and into folders within mobject

mobject/shape_matchers.py

@@ -0,0 +1,56 @@
+from __future__ import absolute_import
+
+from constants import *
+
+from mobject.geometry import Rectangle
+from mobject.geometry import Line
+from mobject.types.vectorized_mobject import VGroup
+from utils.config_ops import digest_config
+from utils.color import Color
+
+class SurroundingRectangle(Rectangle):
+    CONFIG = {
+        "color" : YELLOW,
+        "buff" : SMALL_BUFF,
+    }
+    def __init__(self, mobject, **kwargs):
+        digest_config(self, kwargs)
+        kwargs["width"] = mobject.get_width() + 2*self.buff
+        kwargs["height"] = mobject.get_height() + 2*self.buff
+        Rectangle.__init__(self, **kwargs)
+        self.move_to(mobject)
+
+class BackgroundRectangle(SurroundingRectangle):
+    CONFIG = {
+        "color" : BLACK,
+        "stroke_width" : 0,
+        "fill_opacity" : 0.75,
+        "buff" : 0
+    }
+    def __init__(self, mobject, **kwargs):
+        SurroundingRectangle.__init__(self, mobject, **kwargs)
+        self.original_fill_opacity = self.fill_opacity
+
+    def pointwise_become_partial(self, mobject, a, b):
+        self.set_fill(opacity = b*self.original_fill_opacity)
+        return self
+
+    def set_color(self):
+        # Can't be changin' me!
+        return self
+
+    def get_fill_color(self):
+        return Color(self.color)
+
+class Cross(VGroup):
+    CONFIG = {
+        "stroke_color" : RED,
+        "stroke_width" : 6,
+    }
+    def __init__(self, mobject, **kwargs):
+        VGroup.__init__(self, 
+            Line(UP+LEFT, DOWN+RIGHT),
+            Line(UP+RIGHT, DOWN+LEFT),
+        )
+        self.replace(mobject, stretch = True)
+        self.set_stroke(self.stroke_color, self.stroke_width)