[N64] CPU-based rendering flickers

[thekovic]

Describe the bug
Any rendering performed purely on the CPU, without RDP, displays in ares with severe flickering. This can be observed in libdragon-based homebrew rendering with the graphics module and also with libultra commercial games that draw on the CPU, like Namco Museum 64.

To Reproduce
Steps to reproduce the behavior:

1. Load an N64 ROM that draws using the CPU (for example, Namco Museum 64 or vtest from the libdragon examples).
2. Observe flickering.

Expected behavior
Stable and persistent image without flickering.

Videos
Vtest (libdragon):

n64_cpu_flicker.mp4

Namco Museum 64 (libultra):

n64_cpu_flicker2.mp4