Add IFileSystem::createMemoryWriteFile() to allow writing to memory.
Replace CMemoryReadFile with CMemoryFile that also implement IWriteFile.
Add an IVideoDriver::writeImageToFile() overload that takes an IWriteFile.
This allows writing (e.g.) screenshots to memory, rather than directly to file.
New unit test added to test the new functionality. Tested on Windows with VS2005 and C::B. I'll do Linux ASAP.
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2081 dfc29bdd-3216-0410-991c-e03cc46cb475