Getting a pointer to JavaVM

[bl1nch]

Some libraries require a pointer to the JavaVM to function properly. So it would be nice to be able to get a pointer to the JavaVM using pyjnius.
For example, to support hardware decoding in ffmpeg, libavcodec requires passing a pointer to the JavaVM using the av_jni_set_java_vm function (documentation: https://ffmpeg.org/doxygen/trunk/jni_8h.html).