[FR] GSM: Support forcing 240p

[TheGershon]

The recent addition of forced GSM modes lacks options for 240p, which is beneficial for certain games (mostly ports) when played on CRT that normally run at 480i.
This can be usually done via a hex edit or through GSM 0.23, and would be a very nice addition to neutrino.

[rickgaiser]

interlaced field mode
The display buffer is full height. Normally 448 for NTSC and 512 for PAL.
Only the even or odd lines are displayed in interlaced mode.
This mode can be forced to progressive, resulting in better image quality.

interlaced frame mode
The display buffer is half height. Normally 224 for NTSC and 256 for PAL.
All lines are displayed for both the even and the odd frames.
This mode can be:

• line doubled to 480p/576p, but that looks ugly
• forced to 240p/288p "progressive" mode, that looks great on a CRT that supports it.

In both cases this only works if the game uses a low resolution. Any good game would refresh the buffer 60/50 times per second. Depending on if the frame is even or odd, the refreshed lines would contain even or odd information.
If a game does this, then making the image progressive would couse it to look 'jumpy'. Jumping 1 line up/down every frame.

I think it's a good idea, but I have to look how to best implement this.