pbsplash: dot wave is sometimes too enthusiastic

Filing a bug so that this isn't forgotten:

me: towards the end of the boot I saw that issue where the wavy dots move very fast for like 3-5 seconds before pbsplash is killed.

Caleb: Regarding the weird speedup issue, im guessing it's something to do with how vsync is handled by fbdev emulation, the tfb_flush_fb() call should block but i guess when the shell starts to launch there's some weird interference between the framebuffer and DRM frontends. It should be fairly straightforward to workaround by doing framerate limiting in pbsplash itself, I'll try implementing that

Thread: !2878 (comment 1280218011)