linuxfb platform should support qpf2 pre-rendered fonts

I am testing Qt on linux for an embedded project and was unable to get pre-rendered (qpf2) fonts to work. An inspection of the code showed that qpf2 font files are only loaded by the QPlatformFontDatabase class which is the base class for other font databases. The linuxfb platform uses the QGenericUnixFontDatabase which inherits from QPlatformFontDatabase but overrides the base class behavior such that qpf2 is no longer supported.

It would be nice to be able to use both the linuxfb platform and pre-rendered fonts.