Doodle improvements 2 (#176)

* Fix some doodle bugs and added Background color functionality

* added protections against accidental doodle erase, screen size changing

* resolve react warning about 'selected' on <option>

app/javascript/mastodon/reducers/compose.js

@@ -65,7 +65,9 @@ const initialState = ImmutableMap({
   doodle: ImmutableMap({
     fg: 'rgb(  0,    0,    0)',
     bg: 'rgb(255,  255,  255)',
+    swapped: false,
     mode: 'draw',
+    size: 'normal',
     weight: 2,
     opacity: 1,
     adaptiveStroke: true,