Stay in floats as much as possible in SkTileGrid, particularly in insert.

Stay in floats as much as possible in SkTileGrid, particularly in insert.

SkTileGrid::insert() is about 15% of recording time before
this CL, which reduces it to ~10%.  Next steps are looking
into some of the TODOs I've left myself, and vectorizing
the math.

Most of the win here comes from converting integer
divisions into float multiplies.

BUG=skia:1021

Review URL: https://codereview.chromium.org/615853007