Add update() to NanoImage

[Simon-L]

Following suggestions on adding this, I added an update() method to the NanoImage class.

Inspired from: memononen/nanovg#269 (comment)
Possible example usage in the Meters UI example: https://gist.github.com/Simon-L/65ac25c8caddebf11190c70b9dfea8a7#file-exampleuimeters-cpp-L226-L252

[Simon-L]

This PR causes an issue somewhere that breaks the texture update in certain circumstances. One case found for now is when adding the NanoButton class found in the FileHandling example
Here's a minimal example reproducing the issue: https://github.com/Simon-L/DPF-minimal-nanoimage-update

[falkTX]

Added in 5d3e0ad in a more safe way

[Simon-L]

Thank you so much!