DM: Draw image frames in a grid

Instead of drawing a large vertical strip for CodecSrc::kAnimated_Mode,
draw a grid of images. Compute the grid based on the square root of the
frame count, so the grid will be roughly square.

BUG=skia:5881

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2445863002

Review-Url: https://codereview.chromium.org/2445863002