Check out third_party/wuffs' "release/c" mirror

This is similar to Chromium's https://crrev.com/c/3086053 commit. Skia
only uses the C form of Wuffs-the-library and not other parts of Wuffs.
The wuffs-mirror-release-c repository excludes everything but that C
form, so it is much smaller and changes much less frequently.

Change-Id: I0d996a4ab52464f64a30345a8431c1fd45e31463
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/439536
Auto-Submit: Nigel Tao <nigeltao@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>

DEPS

@@ -41,7 +41,7 @@ deps = {
   "third_party/externals/spirv-tools"     : "https://skia.googlesource.com/external/github.com/KhronosGroup/SPIRV-Tools.git@11cd875ed88484f93943071083b4821b4c3d2193",
   "third_party/externals/swiftshader"     : "https://swiftshader.googlesource.com/SwiftShader@526b987888fbeb5c952912d41265156d53e9e849",
   #"third_party/externals/v8"              : "https://chromium.googlesource.com/v8/v8.git@5f1ae66d5634e43563b2d25ea652dfb94c31a3b4",
-  "third_party/externals/wuffs"           : "https://skia.googlesource.com/external/github.com/google/wuffs.git@1b9a6cb85b9f359595bd264d4c95da5d1291adc5",
+  "third_party/externals/wuffs"           : "https://skia.googlesource.com/external/github.com/google/wuffs-mirror-release-c.git@600cd96cf47788ee3a74b40a6028b035c9fd6a61",
   "third_party/externals/zlib"            : "https://chromium.googlesource.com/chromium/src/third_party/zlib@c876c8f87101c5a75f6014b0f832499afeb65b73",
 
   "../src": {