Allow caller to control when screen is updated

[gilesp1729]

Added two new calls startBuffering() and endBuffering(). By bracketing a sequence of GFX calls between these, the endWrite() is deferred to the end of the sequence and the display is only updated from the buffer once, rather than after every call.
This allows caller to remove flickering and draw smoothly, especially in the common case when the first call is to clear the screen.

Code that does not use these new routines works as before (there is an endWrite at the end of each GFX call)

Missing endWrites mentioned in PR #3 are incorporated here as well.

[CLAassistant]

All committers have signed the CLA.

[gilesp1729]

I certainly am a Github user, and have signed the CLA, but this check keeps coming up on this PR (not on others I have, though)

[per1234]

Hi @gilesp1729. You can see the email address that is associated with the commits here:

https://api.github.com/repos/arduino-libraries/Arduino_GigaDisplay_GFX/commits/f37a8f23628d864c593ee5c09f9e3faa30cc2690

As the bot explained, that email address must be associated with your GitHub account. The email address is not currently associated with your GitHub account. This is how the CLA system verifies that the author of the commit (which might be different from the person who submitted a pull request) has signed the CLA.

You can learn how to add the email address to your GitHub account from this guide:

https://docs.github.com/en/account-and-profile/setting-up-and-managing-your-personal-account-on-github/managing-email-preferences/adding-an-email-address-to-your-github-account

not on others I have, though

The CLA check is something Arduino has set up in our repositories. When you submit PRs to repositories owned by other organizations or individuals, they might not have a CLA, or they might not have an automated system to check it.

[KurtE]

@gilesp1729 @per1234 in response to a user on the forum, I was about to suggest that something like this be
added to the library.

https://forum.arduino.cc/t/giga-shield-analog-display-updates/1302648

Was going to implement something like this and potentially a PR, but then I still have a pending bug fix PR from Feb.
So was not sure...