add SkColorSpace param to SkPixmap::erase()

We don't have a way to erase with a non-sRGB color.

Update the P3 gm to test this, removing the SkBitmap erase case where
there's no option for even an SkColor4f, let alone non-sRGB.  I'm not
sure it's really important to have one when we've got this on pixmap.

Updated release notes.

Change-Id: Ie98270d3f83e041593b4c6b2da8e325b36d4ff18
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/285341
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed