add conservative bounds to raster tiling

This allows the tiler to optimally visit only the tiles that might intersect
the drawing. Not all call-sites can cheaply compute their bounds, so for those
we just pass nullptr, which tells the tiler to visit all of the tiles.

Bug: 818693
Bug: 820245
Bug: 820470
Change-Id: I8bda668a99bcdb2a9a74a8278ec0cf1004acba6e
Reviewed-on: https://skia-review.googlesource.com/119570
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
2 files changed