Fix fmode 1 and 2

[kwn]

Summary

Add proper AGA FMODE=1/2 data fetch setup for simplebuffer.

Details

Previously, simplebuffer only special-cased FMODE=3, while FMODE=1 and FMODE=2 still used the default ECS/OCS-style data fetch window. On AGA, FMODE=1/2 use 32-bit bitplane fetches, so the data fetch stop position and scroll compensation need to differ from FMODE=0.

This change:

• Adds FMODE=1/2 handling when calculating DDFSTOP.
• Uses the 32-bit fetch window for FMODE=1/2, e.g. 320px lowres now resolves to DDFSTOP=$00c8 instead of $00d0.
• Adjusts the extra fetch, modulo, and bitplane offset used by X-scrolled simplebuffer displays from 2-byte to 4-byte units.
• Preserves the existing FMODE=0 and FMODE=3 behavior.

Test

Tested with an AGA-enabled ACE project using a 320px lowres simplebuffer viewport in FMODE=1.

Before the fix, the display was visibly skewed/blurred with diagonal line offsets. After the fix, the title/menu screen renders correctly.

Before

After