compositor: also block softpipe for GLX vsync
On some architectures llvmpipe is currently unusable (e.g. RISC-V and LoongArch), and in this case the fallback software Gallium renderer could be softpipe instead of llvmpipe.
Block softpipe too for GLX vsync.
Signed-off-by: Icenowy Zheng uwu@icenowy.me