Add ImGui::DragCanvas, to simplify several custom widgets Bug: skia: Change-Id: Ic44ab00af1117a6bbc17df7f5005f4db707cb5c1 Reviewed-on: https://skia-review.googlesource.com/c/191002 Reviewed-by: Ben Wagner <bungeman@google.com> Commit-Queue: Brian Osman <brianosman@google.com>