Fix bug in erosion filtering

Noticed this while testing out the "don't reuse scratch textures" patch. Prior to this patch, when the erosion code received a scratch texture larger then necessary the bottom was being over-eroded. This CL fills the filter padding area with white in this case to prevent the over erosion. (When scratch textures aren't reused we always get an exactly sized texture so the old clear to black was having no effect).

[Stephen White, 2013-10-29 13:56:30]

On 2013/10/29 13:45:22, robertphillips wrote:

Good catch!

LGTM

[robertphillips, 2013-10-29 13:59:45]

committed as r11966

[Stephen White, 2013-10-29 14:06:19]

Although now that I think about it, this is part of a larger class of problems:
morphology shouldn't be sampling outside of the crop rect, in either the source
image or the temporaries. I've filed
https://code.google.com/p/skia/issues/detail?id=1766 for that.