p96gfx: Screens with height less than default (e.g. 800x500 instead of 800x600 display trash at the bottom

Tested with Z3660 800x600x24 in Copperline. <img width="723" height="625" alt="Image" src="https://github.com/user-attachments/assets/4b4f700f-2316-4716-bdce-8e8e11780992" /> **To Reproduce** Steps to reproduce the behavior: 1. Go to ScreenMode 2. Uncheck "Default" on Height 3. Enter a value smaller than the default 4. Click Use **Architecture** - Amiga (Copperline) **CPU** - m68k **Version** git head