linux: fmpeg patching support (#1415)

batteries included!
updates ffmepg too

alvr/xtask/src/dependencies.rs

@@ -104,14 +104,14 @@ pub fn build_ffmpeg_linux(nvenc_flag: bool) {
     let download_path = afs::deps_dir().join("linux");
     command::download_and_extract_zip(
         &sh,
-        "https://codeload.github.com/FFmpeg/FFmpeg/zip/n4.4",
+        "https://codeload.github.com/FFmpeg/FFmpeg/zip/n5.1",
         &download_path,
     )
     .unwrap();
 
     let final_path = download_path.join("ffmpeg");
 
-    fs::rename(download_path.join("FFmpeg-n4.4"), &final_path).unwrap();
+    fs::rename(download_path.join("FFmpeg-n5.1"), &final_path).unwrap();
 
     let flags = [
         "--enable-gpl",
@@ -205,6 +205,10 @@ pub fn build_ffmpeg_linux(nvenc_flag: bool) {
             .unwrap();
     }
 
+    // Patches ffmpeg for workarounds and patches that have yet to be unstreamed
+    let ffmpeg_command = "for p in ../../../patches/*; do patch -p1 < $p; done";
+    cmd!(sh, "bash -c {ffmpeg_command}").run().unwrap();
+
     let nproc = cmd!(sh, "nproc").read().unwrap();
     cmd!(sh, "make -j{nproc}").run().unwrap();
     cmd!(sh, "make install").run().unwrap();

patches/FFmpeg-devel-v3-vaapi_encode_h264-Only-set-pic_order_cnt_type-to-0-with-B-frames.diff

@@ -0,0 +1,28 @@
+diff --git a/libavcodec/vaapi_encode_h264.c b/libavcodec/vaapi_encode_h264.c
+index f15bcc6..de0951f 100644
+--- a/libavcodec/vaapi_encode_h264.c
++++ b/libavcodec/vaapi_encode_h264.c
+@@ -361,8 +361,10 @@ static int vaapi_encode_h264_init_sequence_params(AVCodecContext *avctx)
+     sps->chroma_format_idc    = 1;
+
+     sps->log2_max_frame_num_minus4 = 4;
+-    sps->pic_order_cnt_type        = 0;
+-    sps->log2_max_pic_order_cnt_lsb_minus4 = 4;
++    sps->pic_order_cnt_type        = ctx->max_b_depth ? 0 : 2;
++    if (sps->pic_order_cnt_type == 0) {
++        sps->log2_max_pic_order_cnt_lsb_minus4 = 4;
++    }
+
+     sps->max_num_ref_frames = priv->dpb_frames;
+
+@@ -643,6 +645,10 @@ static int vaapi_encode_h264_init_picture_params(AVCodecContext *avctx,
+         }
+     }
+     hpic->pic_order_cnt = pic->display_order - hpic->last_idr_frame;
++    if (priv->raw_sps.pic_order_cnt_type == 2) {
++        hpic->pic_order_cnt *= 2;
++    }
++
+     hpic->dpb_delay     = pic->display_order - pic->encode_order + ctx->max_b_depth;
+     hpic->cpb_delay     = pic->encode_order - hpic->last_idr_frame;
+