This keeps our Rust crates on the same allocator in shared builds.
This fixes the same ELF debug crash as #9026, where mimalloc detects
heap corruption as soon as liblagom-regex.so starts using the first
allocator it finds.
When generating liblagom-regex.so during shared-library linkage, the
normal linker sees that those allocator symbols are already provided by
liblagom-unicode.so, which it depends on, so the corresponding object
file is not pulled in from libregex_rust.a.
Fast forward to runtime, if liblagom-gfx.so is loaded first, as seems
to happen in debug builds, then its exported allocator is picked up by
liblagom-regex.so instead.
Using the Rust yuv crate, eagerly convert from YUV to RGBA on the CPU
when a GPU context is unavailable.
Time spent converting an 8-bit YUV frame with this crate is better than
libyuv on ARM by about 20%, and on x86 with AVX2, it achieves similar
numbers to libyuv.
