[base] Clean up stream reading.

* src/base/ftstream.c (FT_Stream_ReadUShort, FT_Stream_ReadUOffset,
FT_Stream_ReadULong and their LE variants): Remove unnecessary
initialization and slightly refactor.
(FT_Stream_GetByte, FT_Stream_ReadByte): Rename to return unsigned
value and align with sister functions.

* include/freetype/internal/ftstream.h (FT_Stream_GetByte,
FT_Stream_ReadByte): Update prototypes and caller macros.
2 files changed