avoid chrome webworker OOM bug; closes #124

chrome (and chromium-based browsers) can OOM when:

* the OS is Windows, MacOS, or Android (but not Linux?)
* the website is hosted on a remote IP (not localhost)
* webworkers are used to read files

unfortunately this also applies to Android, which heavily relies
on webworkers to make read-speeds anywhere close to acceptable

as for android, there are diminishing returns with more than 4
webworkers (1=1x, 2=2.3x, 3=3.8x, 4=4.2x, 6=4.5x, 8=5.3x), and
limiting the number of workers to ensure at least one idle core
appears to sufficiently reduce the OOM probability

on desktop, webworkers are only necessary for hashwasm, so
limit the number of workers to 2 if crypto.subtle is available
and otherwise use the nproc-1 rule for hashwasm in workers

bug report: https://issues.chromium.org/issues/383568268

README.md

@@ -339,6 +339,9 @@ same order here too
 
 * [Chrome issue 1352210](https://bugs.chromium.org/p/chromium/issues/detail?id=1352210) -- plaintext http may be faster at filehashing than https (but also extremely CPU-intensive)
 
+* [Chrome issue 383568268](https://issues.chromium.org/issues/383568268) -- filereaders in webworkers can OOM / crash the browser-tab
+  * copyparty has a workaround which seems to work well enough
+
 * [Firefox issue 1790500](https://bugzilla.mozilla.org/show_bug.cgi?id=1790500) -- entire browser can crash after uploading ~4000 small files
 
 * Android: music playback randomly stops due to [battery usage settings](#fix-unreliable-playback-on-android)

copyparty/web/up2k.js

@@ -1359,7 +1359,15 @@ function up2k_init(subtle) {
             draw_each = good_files.length < 50;
 
         if (WebAssembly && !hws.length) {
-            for (var a = 0; a < Math.min(navigator.hardwareConcurrency || 4, 16); a++)
+            var nw = Math.min(navigator.hardwareConcurrency || 4, 16);
+
+            if (CHROME) {
+                // chrome-bug 383568268 // #124
+                nw = Math.max(1, (nw > 4 ? 4 : (nw - 1)));
+                nw = (subtle && !MOBILE && nw > 2) ? 2 : nw;
+            }
+
+            for (var a = 0; a < nw; a++)
                 hws.push(new Worker(SR + '/.cpr/w.hash.js?_=' + TS));
 
             if (!subtle)