ext/libgav1: Set MAX_BITDEPTH to 12

This allows libavif to decode 12-bit AVIF images using libgav1.

libgav1's cmake (as of writing [1]) defaults to a MAX_BITDEPTH of
10 if not set explicitly.

[1] https://chromium.googlesource.com/codecs/libgav1/+/0fb779c1e169fe6c229cd1fa9cc6ea6feeb441da/cmake/libgav1_build_definitions.cmake#144
diff --git a/ext/libgav1.cmd b/ext/libgav1.cmd
index 284d0de..b7aafea 100755
--- a/ext/libgav1.cmd
+++ b/ext/libgav1.cmd
@@ -15,6 +15,6 @@
 mkdir build
 cd build
 
-cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DLIBGAV1_THREADPOOL_USE_STD_MUTEX=1 -DLIBGAV1_ENABLE_EXAMPLES=0 -DLIBGAV1_ENABLE_TESTS=0 ..
+cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DLIBGAV1_THREADPOOL_USE_STD_MUTEX=1 -DLIBGAV1_ENABLE_EXAMPLES=0 -DLIBGAV1_ENABLE_TESTS=0 -DLIBGAV1_MAX_BITDEPTH=12 ..
 ninja
 cd ../..
diff --git a/ext/libgav1_android.sh b/ext/libgav1_android.sh
index ba4d387..a4340f3 100755
--- a/ext/libgav1_android.sh
+++ b/ext/libgav1_android.sh
@@ -32,6 +32,7 @@
     -DLIBGAV1_THREADPOOL_USE_STD_MUTEX=1 \
     -DLIBGAV1_ENABLE_EXAMPLES=0 \
     -DLIBGAV1_ENABLE_TESTS=0 \
+    -DLIBGAV1_MAX_BITDEPTH=12 \
     -DANDROID_ABI=${abi}
   ninja
   cd ..
diff --git a/tests/docker/build.sh b/tests/docker/build.sh
index 36fe315..c3b8128 100644
--- a/tests/docker/build.sh
+++ b/tests/docker/build.sh
@@ -48,7 +48,7 @@
 cd libgav1
 mkdir build
 cd build
-cmake -G Ninja -DCMAKE_INSTALL_PREFIX=/usr -DBUILD_SHARED_LIBS=1 -DCMAKE_BUILD_TYPE=Release -DLIBGAV1_THREADPOOL_USE_STD_MUTEX=1 -DLIBGAV1_ENABLE_EXAMPLES=0 -DLIBGAV1_ENABLE_TESTS=0 ..
+cmake -G Ninja -DCMAKE_INSTALL_PREFIX=/usr -DBUILD_SHARED_LIBS=1 -DCMAKE_BUILD_TYPE=Release -DLIBGAV1_THREADPOOL_USE_STD_MUTEX=1 -DLIBGAV1_ENABLE_EXAMPLES=0 -DLIBGAV1_ENABLE_TESTS=0 -DLIBGAV1_MAX_BITDEPTH=12 ..
 ninja install
 
 # rav1e