app: always send paint event when bumping gen
app: always send paint event when bumping gen

When the generation number is bumped, the <-endPaint case will drop
any old paints. So if a new paint isn't sent to replace it, the app
locks up.

This is a bug that looks like it crept into cl/12533 just before it
was submitted. (Sorry.) I'm working on tests as part of adding
android orientation support.

Change-Id: I6b62bdc6774e090b67a5577c3eae7355e3937894
Reviewed-on: https://go-review.googlesource.com/13445
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

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