libobs: Add texture-based encoding support

Allows the ability to encode by passing NV12 textures.  This uses a
separate thread for texture-based encoders with a small queue of
textures.  An output texture with a keyed mutex shared texture is locked
between OBS and each encoder.  A new encoder callback and capability
flag is used to encode with textures.

libobs/CMakeLists.txt

@@ -384,6 +384,7 @@ set(libobs_libobs_SOURCES
 	obs-view.c
 	obs-scene.c
 	obs-audio.c
+	obs-video-gpu-encode.c
 	obs-video.c)
 set(libobs_libobs_HEADERS
 	${libobs_PLATFORM_HEADERS}