buffer: only cl_cache flush once for 1x1 get calls
buffer: change nearest fast path
buffer: disable gegl_buffer_lock / gegl_buffer_unlock

Without threading; the locking is just function call overhead.
buffer: remove one conditional in 1x1px get
buffer: avoid typecheck in gegl_buffer_sample
perf: add missing perf-build.sh file
perf: make build process more resilient