Add an msaa flag to CanvasKit viewer

Adds a simple system for the user to supply flags via the location hash.

  e.g., "http://.../viewer.html#msaa:8"

Implements the msaa flag by rendering to a multisampled offscreen
framebuffer, then blitting it to the main canvas framebuffer.

Change-Id: I7f2b8b769e491f2169fd6b967a72a8ea9c8ffb8b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/292199
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed