Change GaussianBlur logic wrt color spaces

I was using 'source' where I should have said 'input'. Also, to be
consistent with other image filters, ensure that the input is in the
destination gamut before we start blurring.

BUG=skia:

Change-Id: I751961b42a2a5d110ee8ea8916279c8fe0d5248e
Reviewed-on: https://skia-review.googlesource.com/6486
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed