Cache decoded toga color on gtk (#3117)

Cache decoded color on GTK backend.

Author: proneon267

changes/3117.misc.rst

@@ -0,0 +1 @@
+On GTK, native colors representations are now cached.

gtk/src/toga_gtk/colors.py

@@ -6,5 +6,6 @@ def native_color(c):
         color = CACHE[c]
     except KeyError:
         color = (c.rgba.r / 255, c.rgba.g / 255, c.rgba.b / 255, c.rgba.a)
+        CACHE[c] = color
 
     return color

gtk/src/toga_gtk/widgets/button.py

@@ -36,9 +36,7 @@ def set_enabled(self, value):
 
     def set_background_color(self, color):
         # Buttons interpret TRANSPARENT backgrounds as a reset
-        if color == TRANSPARENT:
-            color = None
-        super().set_background_color(color)
+        super().set_background_color(None if color is TRANSPARENT else color)
 
     def rehint(self):
         # print(