Fix ImTextureID type #456

Job	Run time
clang-format	9s
Windows MSVC Shared Release	2m 11s
Windows MSVC Shared Debug	1m 43s
Windows MSVC Static Release	3m 10s
Windows MSVC Static Debug	3m 2s
Windows ClangCL Shared Release	2m 14s
Windows ClangCL Shared Debug	2m 33s
Windows ClangCL Static Release	4m 26s
Windows ClangCL Static Debug	4m 36s
Windows Clang Shared Release	2m 8s
Windows Clang Shared Debug	1m 21s
Windows Clang Static Release	2m 2s
Windows Clang Static Debug	1m 47s
Linux GCC Shared Release	48s
Linux GCC Shared Debug	49s
Linux GCC Static Release	1m 9s
Linux GCC Static Debug	1m 4s
Linux Clang Shared Release	59s
Linux Clang Shared Debug	57s
Linux Clang Static Release	1m 44s
Linux Clang Static Debug	1m 27s
macOS Clang Shared Release	1m 0s
macOS Clang Shared Debug	1m 10s
macOS Clang Static Release	1m 1s
macOS Clang Static Debug	45s
	44m 15s

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