QFontDatabase::smoothSizes() wrong for PCF files

Found when running the QFontDialog test on OpenSuse, which has a lot of PCF bitmap fonts. QFontDatabase::smoothSizes() for the fonts (such as Adobe Utopia) can contain non-supported point sizes, which leads to the font dialog having populated the size list with unsupported sizes. If the first size is unsupported, then no size will be preselected, and the test will fail.