eframe: defer graphics state initialization until app Resumed

Conceptually the Winit `Resumed` event signifies that the application is
ready to run and render and since
rust-windowing/winit#2331 all platforms now
consistently emit a Resumed event that can be used to initialize
graphics state for an application.

On Android in particular it's important to wait until the application
has Resumed before initializing graphics state since it won't have an
associated SurfaceView while paused.

Without this change then Android applications are likely to just show
a black screen.

This updates the Wgpu+Winit and Glow+Winit integration for eframe but
it's worth noting that the Glow integration is still not able to fully
support suspend and resume on Android due to limitations with Glutin's
API that mean we can't destroy and create a Window without also
destroying the GL context, and therefore (practically) the entire
application.

There is a plan (and progress on) to improve the Glutin API here:
rust-windowing/glutin#1435 and with that change
it should be an incremental change to enable Android suspend/resume
support with Glow later.

In the mean time the Glow changes keep the implementation consistent
with the wgpu integration and it should now at least be possible to
start an Android application - even though it won't be able to suspend
correctly.

Author: rib