Move ImGui support code to ImGuiLayer

Viewer still has plenty of code that uses ImGui to create application
specific UI, but the structural code that forwards input to ImGui, and
converts per-frame ImGui rendering data to Skia draw commands is now in
a single component that can be reused in any sk_app-based application.

Bug: skia:
Change-Id: Ic14ece659d4af8ee13b69c638bdaf7df6c24f5c0
Reviewed-on: https://skia-review.googlesource.com/82627
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
7 files changed