The `MetricsVariations` `FT_Size_Reset_Func` is defined to take an
`FT_Size`. Because `tt_size_reset_height` is to be used as such a
function, it must also take an `FT_Size` instead of a `TT_Size`. Even
though the pointers passed will be the same at runtime, calling a
function through a pointer of a different type from the original
function pointer type is undefined behavior. This may be caught at
runtime by Control Flow Integrity with something like clang's
`cfi-icall`.
Issue: https://crbug.com/1433651
* src/truetype/ttobjs.h (tt_size_reset_height): take `FT_Size`
* src/truetype/ttobjs.c (tt_size_reset_height): take `FT_Size` and
update documentation
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