#1260:

* probe which encodings are available at runtime
* modularize the Encoder code so we can query device and codec support without doing a full context init (which requires the codec to be specified!)
* add guards to all functions to prevent NULL dereference if we call a function without a valid context / device initialized

git-svn-id: https://xpra.org/svn/Xpra/trunk@13363 3bb7dfac-3a0b-4e04-842a-767bc560f471