Vector media buttons

[hsaturn]

• Describe the scope of your change--i.e. what the change does and what parts
  of the code were modified. This will help us understand any risks of integrating
  the code.

Instead of having hard-coded coordinates, let's compute the coords.
I was tired of seeing too large buttons for my 0.96 oled display.

The risk is to have mediabuttons bad or not displayed. No other impact.

• Describe any known limitations with your change. For example if the change
  doesn't apply to a supported platform of the library please mention it.

I've removed the delay(...) as I really do not see what this gives to the demo.

• Please run any tests or examples that can exercise your modified code. We
  strive to not break users of the code and running tests/examples helps with this
  process.

There is no risk at all that that PR breaks any existing code. (only the example itself)