Create Debugger slide for Viewer.

This is a very rough first draft which provides the bare minimum. Shader
source is dumped to the window as static text, along with an arrow
tracking the current trace line. Two buttons allow for single-step and
step-over. There aren't yet any GUI affordances for the stack or
variables.

http://screen/44WAnxchjy8MPjM

Change-Id: Ifad4a146b54d334113b02132eec2238af4fd8580
Bug: skia:12666
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/481681
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed