fix f16 -> sRGB in encode_bitmap_for_png()

The existing logic looks wrong to me:
   - clamp premul to [0,1]
   - unpremul, ignoring zero alpha

It seems like we should do:
   - unpremul, avoiding any divide by zero
   - clamp unpremul to [0,1]

Am I misunderstanding this or has this just always been wrong?

Change-Id: I9636b9566c746bc05371e1e660f4e59dde16827b
Reviewed-on: https://skia-review.googlesource.com/19264
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed