Replace it with a function-local static that is initialized on first
use. This gets rid of a global variable and removes the need for
manual initialization in UICommon.
This commit also replaces the weird find_if that looks for a non-null
unique_ptr with a simple "is vector empty" check considering that
none of the pointers can be null by construction.
It already is disabled for other backends, but this didn't happen with the software renderer. Attempting to change it while running causes the change to visually happen (including switching to the normal render settings UI instead of the barren one for the software renderer), but doesn't actually change the backend itself (it'll still use the software renderer at the next launch).
