Fix drag events in Mac viewer.

Rather than depend on the Cocoa event system we intercept the events
in our main loop and send them to the appropriate sk_app::Window. A
hashmap that maps from Cocoa windowNumbers to an sk_app::Window is
added to make this possible.

We continue to send the event on through the Cocoa system to catch
system level events -- e.g., window close and drag events. We also
continue to catch key events in an NSView to keep the app from beeping
annoyingly because it thinks it's capturing events outside its focus.
Finally we ensure that move events are always enabled for the window
so that imgui knows that the cursor is over it.

Bug: skia:8737
Change-Id: Id49df51f68942fbf51634d6484291df862074864
Reviewed-on: https://skia-review.googlesource.com/c/191574
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed