Draft: Resolve "Exporting to raw *.data files exports 16 bit precision images as 8 bit precision"

Closes #7091 (closed)

The current behavior (in master) is not very consistent with UI and IMO with user expectation.

When user exports the image in the raw data format they are asked about export format:

However in fact the plugin exports the data in the format depending on the current image format: https://gitlab.gnome.org/GNOME/gimp/-/blob/fac84db0289ddc0808c6b5e3b1343eb9df546359/plug-ins/common/file-raw-data.c#L1071

So to user it looks like they select RGB format but in fact it might be saved in grayscale if this is the current image format.

So instead of that I suggest using the user selected option as a format that the image will be exported at the selected format, no matter what current image format is:

This way, even if your current image is RGB 8 bits per pixel and you want to export grayscale 16 bit, you can select the latter and this is what will be used as the export format.

I also added "Use current image format" option which takes the current image format for the export.