Missing Plymouth BGRT fallback image for `fedora-logos` and `generic-logos`
As part of the Fedora Asahi SIG work, @karolherbst and I discovered that we don't have artwork for Plymouth to fallback to when there's no firmware vendor image to display on-screen. For example, on Lenovo laptops booting via UEFI, Fedora's bootsplash shows the Lenovo logo, pulled from the firmware data.
However, when booting via BIOS or on fake UEFI implementations, it shows absolutely nothing because we don't have a fallback image installed to /usr/share/plymouth/themes/spinner/bgrt-fallback.png
(I probably should ask @halfline if we can make this path configurable so that we don't have to store it there, but that's where it needs to be for now...)
Since this is branding, it needs a generic alternative too.