RectangleTransforms for letter tiles

Signed-off-by: John McCutchan <john@johnmccutchan.com>
Merge branch 'master' of https://github.com/johnmccutchan/tilebasedwordsearch

Signed-off-by: John McCutchan <john@johnmccutchan.com>

Conflicts:
	lib/src/board_view.dart
	web/main.dart
Merge branch 'master' of https://github.com/johnmccutchan/tilebasedwordsearch
Don't draw letters for now.

Signed-off-by: John McCutchan <john@johnmccutchan.com>