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.

src/encoder/video/video.c

@@ -0,0 +1,26 @@
+#include "../../../include/encoder/video/video.h"
+#include <assert.h>
+
+bool gsr_video_encoder_start(gsr_video_encoder *encoder, AVCodecContext *video_codec_context, AVFrame *frame) {
+    assert(!encoder->started);
+    bool res = encoder->start(encoder, video_codec_context, frame);
+    if(res)
+        encoder->started = true;
+    return res;
+}
+
+void gsr_video_encoder_copy_textures_to_frame(gsr_video_encoder *encoder, AVFrame *frame) {
+    assert(encoder->started);
+    if(encoder->copy_textures_to_frame)
+        encoder->copy_textures_to_frame(encoder, frame);
+}
+
+void gsr_video_encoder_get_textures(gsr_video_encoder *encoder, unsigned int *textures, int *num_textures, gsr_destination_color *destination_color) {
+    assert(encoder->started);
+    encoder->get_textures(encoder, textures, num_textures, destination_color);
+}
+
+void gsr_video_encoder_destroy(gsr_video_encoder *encoder, AVCodecContext *video_codec_context) {
+    assert(encoder->started);
+    encoder->destroy(encoder, video_codec_context);
+}