Deprecate manual brotli decompression

It doesn't have much benefit but comes with
a maintenance cost as we either need to keep
patching upstream's js implementation for use in
JSO — which also bloats binary size by including
two copies of brotli and its default dict — or
use the C implemntation from JS which requires boilerplate
in both C and JS and also additional JS polyfills to keep
e.g. the fontname extraction working.

The browser's in-built support for Content-Encoding
is more felxible, faster and simpler to use.

Cherry-picked from: libass@e459c8e

README.md

@@ -195,11 +195,16 @@ Adjusting `prescaleFactor`, `prescaleHeightLimit` and `maxRenderHeight` to lower
 the rendering canvas can work around this at the expense of visual quality.
 
 
-### Brotli Compressed Subtitles
-The SubtitleOctopus allow the use of compressed subtitles in brotli format,
-saving bandwidth and reducing library startup time
-
-To use, just run: `brotli subFile.ass` and use the .br result file with the subUrl option
+### Brotli Compressed Subtitles (DEPRECATED)
+Manual support for brotli-compressed subtitles is tentatively deprecated
+and may be removed with the next release.
+
+Instead use HTTP's `Content-Encoding:` header to transmit files compressed and
+let the browser handle decompression before it reaches JSO. This supports more
+compression algorithms and is likely faster.
+Do not use a `.br` file extension if you use `Content-Ecoding:` as this will
+conflict with the still existing manual support which tries to decompress any data
+with a `.br` extension.
 
 ## How to build?
 

src/pre-worker.js

@@ -45,7 +45,13 @@ function isBrotliFile(url) {
         len = url.length;
     }
 
-    return url.endsWith(".br", len);
+    if (url.endsWith(".br", len)) {
+        console.warn("Support for manual brotli decompression is tentatively deprecated and "
+                + "may be removed with the next release. Instead use HTTP's Content-Encoding.")
+        return true;
+    }
+
+    return false;
 }
 
 Module = Module || {};