SkLS now accepting nullptr for diffuse shader and normal source, now accurately handling alpha

This CL's base is the CL for taking in a diffuse shader into SkLS on the API side: https://codereview.chromium.org/2064153002

BUG=skia:5502,skia:5517
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2132113002

Review-Url: https://codereview.chromium.org/2132113002
8 files changed