Fix aga bplcon0 bpu3

[kwn]

Summary

Fix AGA BPLCON0 bitplane count setup for viewports below 8 BPP.

Previously, the AGA path in viewLoad() always set BPU3 (BV(4)) in BPLCON0. This makes hardware interpret the viewport as using an 8-bitplane mode even when the requested viewport depth is lower, e.g. 5 BPP. In practice this can result in a blank screen while the rest of the program continues running.

This change sets BPU3 only when the requested bitplane count actually needs it.

Details

• Preserve the existing lower 3-bit BPP setup via (ubBpp & 0x07) << 12.
• Set BPU3 only when ubBpp & 0x08 is true.
• This keeps AGA viewport setup correct for both lower depths (1-7 BPP) and 8 BPP.

Test

Tested with an AGA-enabled ACE project using a 5 BPP viewport on an A1200 FS-UAE configuration. Before the fix, palette updates and game logic worked, but bitmap/sprite output was blank. After the fix, graphics and sprites render correctly.