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Blood: Fix shadows crashing when rendered in mirrors #918

Job	Run time
Windows MSYS2 x86_64 GCC	10m 19s
Windows MSYS2 i686 GCC	10m 12s
Windows MSYS2 x86_64 GCC without SDL	9m 47s
Linux GCC	4m 54s
Linux GCC SDL 1.2	3m 57s
Linux Clang	6m 0s
Linux Clang without GL	5m 36s
Linux Clang without Polymer	5m 56s
macOS Clang	3m 13s
	59m 54s

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