readFontStream() to load fonts from a std::istream, rather than from the
local filesystem by name. Such a call may be used, for example, if the
user has a font fetched over a network, or a font available in memory
without a correspondng filename.
The changes implement the new function by following the corresponding code
for readFontFile(). readFontStream() reads a stream into memory, and
holds that memory for FreeType.
As a basic test, I mangled the osgtext example to use
readFontStream(std::ifstream("font")) in lieu of a readFontFile call, and
the modified example ran completely."
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