Enable automatic rescaling in bench_pictures

bench_pictures with "--device gpu" is failing because we're trying to allocate
too much GPU memory.  Move the recently-added scaling code into picture_utils
and share it between render_pictures and bench_pictures.
Review URL: https://codereview.appspot.com/6495125

git-svn-id: http://skia.googlecode.com/svn/trunk@5543 2bbb7eff-a529-9590-31e7-b0007b416f81
5 files changed