add display_mut to EmbeddedBackend

[x37v]

This allows users to get the Display and draw to it directly if they'd like.

[j-g00da]

Thanks! Can you add a simple test that draws something using ratatui, then draws dieractly and finally checks the resulting framebuffer?

[x37v]

@j-g00da how is that?

[x37v]

I'm actually trying to use this in my project right now and running into some issues so I'll report back once I have it fixed up

[x37v]

okay.. it does work, my issue was that i was rendering to the screen in the flush_callback but that must happen as part of the terminal.draw call.. now that the display is accessible, i can get rid of my flush_callback and simply tell the terminal to draw, then after, render the Display contents to my physical display.