ensure that partial reads from recv don't trip up rendering #723

Job	Run time
freebsd14 (CMake) (LLVM)	3m 32s
debian:unstable (CMake) (GNU)	3m 15s
fedora:rawhide (CMake) (GNU)	3m 23s
macos-14 (CMake) (LLVM)	6m 28s
ubuntu:devel (CMake) (GNU)	3m 13s
debian:12 (Latest Mapnik) (CMake) (GNU)	14m 44s
ubuntu:24.04 (Autotools) (GNU)	2m 16s
ubuntu:24.04 (Latest Mapnik) (CMake) (GNU)	15m 31s
ubuntu:24.04 (Autotools) (LLVM)	2m 38s
ubuntu:24.04 (CMake) (GNU)	3m 41s
ubuntu:24.04 (CMake) (LLVM)	3m 12s
	1h 1m 53s

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