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Don't accept overlong sequences. Don't accept codepoints past the
maximum unicode codepoint. Don't accept the UTF-16 surrogate codepoints.
I'm not sure if there are more codepoints that are defined to be
invalid, but we just want to make libavcodec happy, so this is enough.
(libavcodec's subtitle converter checks for valid UTF-8 and throws up
and dies if it's not - now we want to use bstr_sanitize_utf8_latin1() to
force valid UTF-8, so the strictness of our UTF-8 parser has to match at
least that of the libavcodec's check.)
I'm not sure whether the min test is actually 100% correct.
Note that libavcodec also treats BOM codepoints as invalid. This is
definitely a bug: the BOM is really just "zero-width non-breaking space"
redefined by Microsoft, but it is perfectly valid to appear in the
middle of a string. Official Unicode has merely deprecated the old
usage of the BOM codepoint, and didn't make it illegal. Besides, the
string could be from the start of a file, so even this check doesn't
make sense even with libavcodec's insane logic. We don't copy this bug.
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