win dw: Fallback to aliased if ClearType 'succeeds' but returns empty
This appears to happen for some CJK fonts where DW only returns valid
data for bitmaps, not for cleartype data.
This has an artifact (of course) which is that during the run where this
failure case is encountered, cleartype will appear incongrously next to
aliased, which looks poor. So, we'll need to fallback at a higher level.
However, for M37, having something visible is more desirable than
rendering blank space for characters.
For reference, two screenshots. M37 Stable and then with this patch:
content of test html file.