Separate video encoding method from capture method

With this instead of kms_cuda/kms_vaapi/kms_software and
xcomposite_cuda/xcomposite_vaapi/xcomposite_software there
is now only kms and xcomposite.

include/encoder/video/vaapi.h

@@ -0,0 +1,15 @@
+#ifndef GSR_ENCODER_VIDEO_VAAPI_H
+#define GSR_ENCODER_VIDEO_VAAPI_H
+
+#include "video.h"
+
+typedef struct gsr_egl gsr_egl;
+
+typedef struct {
+    gsr_egl *egl;
+    bool hdr;
+} gsr_video_encoder_vaapi_params;
+
+gsr_video_encoder* gsr_video_encoder_vaapi_create(const gsr_video_encoder_vaapi_params *params);
+
+#endif /* GSR_ENCODER_VIDEO_VAAPI_H */