NEO direct video memory dump option for screen snapshot

[bbbradsmith]

An alternative screenshot option based on a recent mailing list discussion.

Enabled via Config.bNEOScreenSnapShot (no UI added for it).

This creates a NEO file format screenshot, out of the data in FrameBuffer (screen.c). This works for ST low, medium, and high resolutions.

Otherwise with TT or Falcon the FrameBuffer structure is not filled with any screen data, so it instead dumps data from ST RAM at the current video base address.

The palette is taken from the centre line of the FrameBuffer for Low and Medium resolutions. High resolution and TT/Falcon don't seem to store per-line palette, but I instead extracted the GenConvert palette for it.

Some notes about the .NEO format:

• The original NEOChrome program only works with 320x200 4bpp images, though the format has a few header fields that can support other resolutions.
• There are 3 known resolution values (0=low, 1=med, 2=high). This dump will use one of those 3 values if the data is 4, 2, or 1bpp. As an extension for resolutions with different bpp, I have simply placed the bpp value here to assist identification.
• The internal filename starts with "HATARI" and ends with BPP indicator text (e.g. "4BPP"), which can be used to identify the format extension if needed, and as a hint about the BPP.
• The 16 palette entries in the header will be filled with up to 16 x 9-bit colour values in the standard ST format (or STE 12-bit values). If the machine is not an ST and BPP <= 8, a suffix of 2^bpp x 24-bit RGB palette entries will follow the video data to allow a full recovery of the image's original colour. This suffix will begin with 3 ASCII characters "RGB" for verification. (This suffix is ignored by NEOChrome, so 4bpp 320x200 images will a suffix are still compatible.)
• The screen width and height fields are used, indicating the screen dimensions without borders.
• The amount of data that follows the header will match the screen width, height, and bpp.

[huth]

Since @npomarede merge the NEO support, I think we can close this pull request here now