Make include-what-you-use happy with some files in source/lib and fix
what needs to be fixed.
Ref: #8086
Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Comments by: @nwtour, @Langbart, @bb
Based on patch by: @echotangoecho
Tested on Windows 7 & 10, Ubuntu and macos.
Differential Revision: https://code.wildfiregames.com/D721
This was SVN commit r26093.