DynamicTextureAtlas: fix m_TexArraySize race with monotonic atomic up… #4434

Job	Run time
Linux -> Pre-checks	22s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 27s
Linux -> Clang-x64, Release	13m 35s
Linux -> GCC-x64, Debug	11m 58s
Linux -> GCC-x64, Release	10m 57s
Linux -> Clang-x64, Debug	13m 45s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 57s
	1h 6m 1s

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