MAINT: Downgrade Opus submodule

Since newer Opus versions appear to cause crashes under some
circumstances, we downgrade our Opus submodule to point to the latest
stable release, which is v1.3.1 from 2019.

We will only be able to upgrade again, once the underlying issue causing
the crashes has been identified and fixed.

Fixes #5302

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "3rdparty/opus"]
 	path = 3rdparty/opus
-	url = https://gitlab.xiph.org/xiph/opus.git
+	url = https://github.com/mumble-voip/opus.git
 [submodule "3rdparty/minhook"]
 	path = 3rdparty/minhook
 	url = https://github.com/mumble-voip/minhook.git
@@ -25,3 +25,5 @@
 [submodule "3rdparty/gsl"]
 	path = 3rdparty/gsl
 	url = https://github.com/microsoft/GSL.git
+[submodule "opus"]
+	url = https://github.com/mumble-voip/opus.git

3rdparty/opus

@@ -1 +1 @@
-Subproject commit 6b6035ae4a29abbd237463d84a45fbeb0d92bc18
+Subproject commit 027cd11ed732d02456d1f049081d24b4626d5cb9

docs/dev/build-instructions/cmake_options.md

@@ -139,6 +139,11 @@ Build a heavily optimized version, specific to the machine it's being compiled o
 (No description available)
 (Default: ON)
 
+### OPUS_BUILD_TESTS
+
+(No description available)
+(Default: OFF)
+
 ### OPUS_STACK_PROTECTOR
 
 (No description available)

src/mumble/CMakeLists.txt

@@ -696,6 +696,7 @@ endif()
 
 if(bundled-opus)
 	option(OPUS_BUILD_SHARED_LIBRARY "" ON)
+	option(OPUS_BUILD_TESTS "" OFF)
 	if(MINGW)
 		option(OPUS_STACK_PROTECTOR "" OFF)
 	endif()