--tilesize=<PIXELS> option support

[tyberiuscodes]

First of all, I wanted to thank you for this handy tool. I was using the "vanilla" gdal2tiles, but was having a lot of trouble using it with leaflet. After finding this leaflet version, I was finally able to get my tiles to display properly on the y-axis!

However, I came to realize that gdal2tiles-leaflet unfortunately doesn't have support for the new "--tilesize" option from gdal2tiles 3.1 (see https://gdal.org/programs/gdal2tiles.html#cmdoption-gdal_translate-tilesize). With the original program, I was generating my tiles at 215x215 rather than 256px, since my image is 13760x6880. This means that when I use the leaflet version to generate the tiles, it ends up a little wonky.

And so, I wanted to request the addition of the --tilesize option, if possible. I tried hacking it together myself, but unfortunately I have no experience with python.

Thank you for your time.