Add Optional Read Operations, No DC HWSPI and virtuals

[welshcoder]

Hi,

This PR contains some base changes needed for a potential 2nd PR to add the LT7680 display driver to Arduino_GFX. In summary, these are:

• Add an option to read back from a HWSPI device. I've written it so that by default, it makes no changes to the original library, and is backward compatible with existing projects. However, I've needed to be able to query when the LT7680 is ready to receive the next command. Whilst a fixed delay can be used, I've found this solution to be less than ideal and has caused a problem when the device is not ready or it's just too slow.
• I've added a HWSPI option that does not use a DC pin to control the interpretation of the data being sent. For the LT7680, it always uses 16-bit transfers, but the two most significant bits are used to provide the DC function, and whether it is a read or write function. The least significant byte is used to transfer the actual data. The constructor of the class takes 16-bit values that will be OR'd with the data to set the relevant bits to set the function of subsequent transfers.
• The last change was to make all the drawing functions virtual so that they can all be overridden in derived classes. This was done so that I can use hardware drawing functions of the LT7680 rather than use the Arduino_GFX versions.

For information, the LT7680 datasheet can be found here: https://www.levetop.cn/uploadfiles/2023/05/LT768x_DS_V42_ENG.pdf

I hope you find these edits useful.

[moononournation]

As mentioned in README.md, Arduino_GFX is designed no read operation. You may find other GFX libraries that support read operation and work on it.