refactor(codec): de-Qt oakcodec and wrap it in a pure C ABI; switch common handles to refcounted value structs

- oakcodec: de-Qt all 20 sources (QThread decode loop -> std::thread,
  QObject/signals -> callbacks), pure C ABI in include/codec with
  refcounted neutral handles (OakFrame/OakDecoder/OakEncoder),
  framemanager moved in from render, frame_to_buffer/buffer_to_frame
  moved in from oakcommon oiioutils, codec->task via submit callback
  (M8 will register), all cross-module calls go through the other
  side's C API, -fvisibility=hidden + OAKCODEC_API
- oakcommon: handles become refcounted value structs
  {ctx, addref, release, abi_version} (FFmpeg-style), pass-by-value
  signatures, free() as release wrapper; init_from_native/get_native
  for copyable value objects; OakCommonXxx renamed to OakXxx
- oakcommon: add logging (log_debug/info/warning/critical with level
  filtering and sink injection) + printf-style oakcommon_log C wrapper
- oakrender: add CancelAtom C API family; complete
  oakrender_color_processor_convert_frame; fix get_processor() missing
  definition and OCIO env var lookup
- tests: oakcommon 174, oaknode 96, oakrender 42, oakcodec 18, all
  green in their standalone builds

include/common/error.h

@@ -30,7 +30,7 @@
  * (including the terminating NUL) as a non-negative value instead.
  */
 #define OAKCOMMON_OK 0 /**< Success. */
-#define OAKCOMMON_E_INVALID (-1) /**< NULL handle or invalid argument. */
+#define OAKCOMMON_E_INVALID (-1) /**< Empty handle (ctx == NULL) or invalid argument. */
 #define OAKCOMMON_E_STATE (-2) /**< Call not valid in the current state. */
 #define OAKCOMMON_E_FAILED (-3) /**< The underlying operation failed. */
 #define OAKCOMMON_E_NOT_FOUND (-4) /**< Index out of range / entry not found. */