[base] Reject combinations of incompatible `FT_OPEN_XXX` flags.

The three modes are mutually exclusive, and the documentation of the
`FT_OPEN_XXX` constants notes this.  However, there was no check to
validate this in the code, and the documentation on `FT_Open_Args`
claimed that the corresponding bits were checked in a well-defined
order, implying it was valid (if useless) to specify more than one.
Ironically, this documented order did not agree with the actual
code, so it could not be relied upon; hopefully, nobody did this and
nobody will be hurt by the new validation.

Even if multiple mode bits were allowed, they could cause memory
leaks: if both `FT_OPEN_STREAM` and `stream` are set along with
either `FT_OPEN_MEMORY` or `FT_OPEN_PATHNAME`, then `FT_Stream_New`
allocated a new stream but `FT_Open_Face` marked it as an 'external'
stream, so the stream object was never released.

* src/base/ftobjs.c (FT_Stream_New): Reject incompatible
`FT_OPEN_XXX` flags.
3 files changed