Increase HTML5 THREADPOOL size.

This fixes a "random" deadlock when quitting the editor.
I still haven't figure out the root cause, but having a bigger seems to
greatly mitigate the issue.
The new pool size (pre-allocated threads) is now 8.

platform/javascript/detect.py

@@ -135,7 +135,7 @@ def configure(env):
         env.Append(CPPDEFINES=["PTHREAD_NO_RENAME"])
         env.Append(CCFLAGS=["-s", "USE_PTHREADS=1"])
         env.Append(LINKFLAGS=["-s", "USE_PTHREADS=1"])
-        env.Append(LINKFLAGS=["-s", "PTHREAD_POOL_SIZE=4"])
+        env.Append(LINKFLAGS=["-s", "PTHREAD_POOL_SIZE=8"])
         env.Append(LINKFLAGS=["-s", "WASM_MEM_MAX=2048MB"])
         env.extra_suffix = ".threads" + env.extra_suffix
     else: