Tile: avoid negative position for paint rect in render process
Tile: avoid negative position for paint rect in render process

To avoid missing update, we enlarge paint rects but this generates
negative position to canvas. Fix this.

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser