DynamicTextureAtlas: fix m_TexArraySize race with monotonic atomic up… #4433

Job	Run time
Linux -> Pre-checks	19s
Linux -> Clang-x64, Debug	13m 35s
Linux -> Clang-x64, Release	13m 41s
Linux -> GCC-x64, Debug	12m 10s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 39s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 18s
Linux -> GCC-x64, Release	10m 40s
	1h 5m 22s

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