Split fetching FFmpeg from build_ffmpeg.sh

It makes it harder to work with a different branch. Also make
the FFmpeg instructions consistent with those of the other
extensions (e.g., by removing the "jni" part of the path from
the FFMPEG_EXT_PATH variable.

PiperOrigin-RevId: 310937751

extensions/ffmpeg/README.md

@@ -25,7 +25,7 @@ follows:
 
 ```
 cd "<path to exoplayer checkout>"
-FFMPEG_EXT_PATH="$(pwd)/extensions/ffmpeg/src/main/jni"
+FFMPEG_EXT_PATH="$(pwd)/extensions/ffmpeg/src/main"
 ```
 
 * Download the [Android NDK][] and set its location in a shell variable.
@@ -41,6 +41,21 @@ NDK_PATH="<path to Android NDK>"
 HOST_PLATFORM="linux-x86_64"
 ```
 
+* Fetch FFmpeg:
+
+```
+cd "${FFMPEG_EXT_PATH}/jni" && \
+git clone git://source.ffmpeg.org/ffmpeg ffmpeg
+```
+
+* Checkout an appropriate branch of FFmpeg. We cannot guarantee compatibility
+  with all versions of FFmpeg. We currently recommend version 4.2:
+
+```
+cd "${FFMPEG_EXT_PATH}/jni/ffmpeg" && \
+git checkout release/4.2
+```
+
 * Configure the decoders to include. See the [Supported formats][] page for
   details of the available decoders, and which formats they support.
 
@@ -48,12 +63,12 @@ HOST_PLATFORM="linux-x86_64"
 ENABLED_DECODERS=(vorbis opus flac)
 ```
 
-* Fetch and build FFmpeg. Executing `build_ffmpeg.sh` will fetch and build
-  FFmpeg 4.2 for `armeabi-v7a`, `arm64-v8a`, `x86` and `x86_64`. The script can
-  be edited if you need to build for different architectures.
+* Execute `build_ffmpeg.sh` to build FFmpeg for `armeabi-v7a`, `arm64-v8a`,
+  `x86` and `x86_64`. The script can be edited if you need to build for
+  different architectures:
 
 ```
-cd "${FFMPEG_EXT_PATH}" && \
+cd "${FFMPEG_EXT_PATH}/jni" && \
 ./build_ffmpeg.sh \
   "${FFMPEG_EXT_PATH}" "${NDK_PATH}" "${HOST_PLATFORM}" "${ENABLED_DECODERS[@]}"
 ```
@@ -62,7 +77,7 @@ cd "${FFMPEG_EXT_PATH}" && \
   built in the previous step. For example:
 
 ```
-cd "${FFMPEG_EXT_PATH}" && \
+cd "${FFMPEG_EXT_PATH}/jni" && \
 ${NDK_PATH}/ndk-build APP_ABI="armeabi-v7a arm64-v8a x86 x86_64" -j4
 ```
 

extensions/ffmpeg/src/main/jni/build_ffmpeg.sh

@@ -41,10 +41,7 @@ for decoder in "${ENABLED_DECODERS[@]}"
 do
     COMMON_OPTIONS="${COMMON_OPTIONS} --enable-decoder=${decoder}"
 done
-cd "${FFMPEG_EXT_PATH}"
-(git -C ffmpeg pull || git clone git://source.ffmpeg.org/ffmpeg ffmpeg)
-cd ffmpeg
-git checkout release/4.2
+cd "${FFMPEG_EXT_PATH}/jni/ffmpeg"
 ./configure \
     --libdir=android-libs/armeabi-v7a \
     --arch=arm \

extensions/vp9/README.md

@@ -42,9 +42,8 @@ cd "${VP9_EXT_PATH}/jni" && \
 git clone https://chromium.googlesource.com/webm/libvpx libvpx
 ```
 
-* Checkout the appropriate branch of libvpx (the scripts and makefiles bundled
-  in this repo are known to work only at specific versions of the library - we
-  will update this periodically as newer versions of libvpx are released):
+* Checkout an appropriate branch of libvpx. We cannot guarantee compatibility
+  with all versions of libvpx. We currently recommend version 1.8.0:
 
 ```
 cd "${VP9_EXT_PATH}/jni/libvpx" && \