Add support for transparent backgrounds

[jasper-zanjani]

Although the available choices for background color are great, it does not appear there is a way to explicitly specify a transparent background--that is, the terminal's native background.

[vinayak-mehta]

That's interesting, I'll look into how that can be done :)

[jasper-zanjani]

That's interesting, I'll look into how that can be done :)

Well it looks like the default styling for Slide 2 already does it, at least for areas where there are no characters.

[vinayak-mehta]

Which terminal are you using? And what is the markdown for that slide? (since it isn't in the examples)

I tried setting bg=black on a transparent guake terminal but that didn't seem to work. I'll have to look into how transparency works with asciimatics.

[jasper-zanjani]

I was using Ubuntu in Windows Terminal. Using PowerShell Core in Windows Terminal produces the same effect. I did not specify a background for either slide. The markdown I used for the following images was:

# Testing Present
Slide 1
---
## Continuing test
Slide 2

So you can see, by default the first slide uses the white color, but the second slide does not paint the terminal except for where characters appear. So present is already implementing some form of transparency..