Fix scroller - 6BPP & 7BPP, fix HiRes

[kwn]

Description

Fix scroll buffer display setup for higher bit depths and hires modes.

This PR improves scroll buffer behavior for 6bpp and 7bpp in FMODE 0, 1, and 3. FMODE 3 is usable but still shows minor horizontal black-line glitches.

It also fixes hires mode setup by correctly composing and writing BPLCON0, and simplifies the related BPLCON0 and BPLCON2 logic in src/ace/utils/extview.c.

Known limitation: 8bpp scroll buffer modes are still not fixed. They likely need a deeper change to the scrolling/wrapping mechanism because the current Copper update approach appears to exceed available timing.

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[tehKaiN]

Generally looks good.

I'm not sure whether those extended aga-related ifs/switches in fetchmode.h won't introduce extra overhead for ocs since there's no e.g. ifdef aga in calculating bitplane shift, but if that happens we'll add hot paths.