Document Windows FFmpeg build, etc.

Statically link FFmpeg on Windows. This requires building a slimmed-down
version of FFmpeg from source. The build process is documented in the
README.

Also add ffmpeg dependency to the nix flake.

README.md

@@ -64,6 +64,40 @@ First install MSYS2, and then open the MINGW64 shell and run the following comma
         mingw-w64-x86_64-libsndfile \
         mingw-w64-x86_64-glew
 
+To build with FFmpeg support, you must compile FFmpeg as a static library:
+
+    git clone https://github.com/FFmpeg/FFmpeg.git
+    cd FFmpeg
+    git checkout n6.0
+    ./configure --disable-everything \
+        --enable-decoder=mpegvideo \
+        --enable-decoder=mpeg1video \
+        --enable-decoder=mpeg2video \
+        --enable-decoder=mp2 \
+        --enable-parser=mpegaudio \
+        --enable-parser=mpegvideo \
+        --enable-demuxer=mpegps \
+        --enable-demuxer=mpegts \
+        --enable-demuxer=mpegtsraw \
+        --enable-demuxer=mpegvideo \
+        --enable-decoder=vc1 \
+        --enable-decoder=wmapro \
+        --enable-parser=vc1 \
+        --enable-hwaccel=vc1_d3d11va \
+        --enable-hwaccel=vc1_d3d11va2 \
+        --enable-hwaccel=vc1_dxva2 \
+        --enable-demuxer=asf \
+        --enable-protocol=file \
+        --enable-filter=scale \
+        --enable-static \
+        --disable-shared \
+        --extra-libs=-static \
+        --extra-cflags=--static
+    make
+    make install
+    cd ..
+    export PKG_CONFIG_PATH="/usr/local/lib/pkgconfig:$PKG_CONFIG_PATH"
+
 Then build the xsystem4 executable with meson,
 
     mkdir build

flake.nix

@@ -36,6 +36,7 @@
             chibi
             flex
             bison
+            ffmpeg
           ];
         }
       );
@@ -65,6 +66,7 @@
               chibi
               flex
               bison
+              ffmpeg
             ];
           };
         }

meson.build

@@ -20,10 +20,10 @@ png = dependency('libpng', static : static_libs)
 cglm = dependency('cglm', version : '>=0.8.3', fallback : ['cglm', 'cglm_dep'])
 sndfile = dependency('sndfile', static : static_libs)
 
-avcodec = dependency('libavcodec', required: false)
-avformat = dependency('libavformat', required: false)
-avutil = dependency('libavutil', required: false)
-swscale = dependency('libswscale', required: false)
+avcodec = dependency('libavcodec', required: false, static : static_libs)
+avformat = dependency('libavformat', required: false, static : static_libs)
+avutil = dependency('libavutil', required: false, static : static_libs)
+swscale = dependency('libswscale', required: false, static : static_libs)
 
 gles = dependency('glesv2', version : '>=3', required: get_option('opengles'))
 if gles.found()