Refactor explicit inverse calculation in SkImageShader::onAppendStages()

We have SkShaderBase::computeTotalInverse() which does exactly that.

Change-Id: I77695f9261d700c3ac25fe94250b798e6edfc25d
Reviewed-on: https://skia-review.googlesource.com/105000
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed