Add sw support to viewer on Linux.

This doesn't yet do anything with srgb/colorspace.

Replaces the --vulkan flag with --backend which takes 'gl', 'vk', or 'sw'.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2165813002

Review-Url: https://codereview.chromium.org/2165813002
4 files changed