ExternalStreamManagerAndroid actually depends on GLES

Since ExternalStreamManagerAndroid depends on GLES, move it to the 
opengl directory.

filament/backend/CMakeLists.txt

@@ -87,7 +87,7 @@ if (FILAMENT_SUPPORTS_OPENGL AND NOT FILAMENT_USE_EXTERNAL_GLES3 AND NOT FILAMEN
         list(APPEND SRCS src/VirtualMachineEnv.cpp)
     endif ()
     if (ANDROID)
-        list(APPEND SRCS src/android/ExternalStreamManagerAndroid.cpp)
+        list(APPEND SRCS src/opengl/ExternalStreamManagerAndroid.cpp)
         list(APPEND SRCS src/android/ExternalTextureManagerAndroid.cpp)
         list(APPEND SRCS src/opengl/PlatformEGLAndroid.cpp)
     elseif (IOS)

filament/backend/src/android/ExternalTextureManagerAndroid.h

@@ -31,6 +31,15 @@
 
 namespace filament {
 
+/*
+ * ExternalTextureManagerAndroid::ExternalTexture is basically a wrapper for AHardwareBuffer.
+ *
+ * This class doesn't rely on GL or EGL, and could be used for other Android platform if needed
+ * (e.g. Vulkan).
+ *
+ * ExternalTextureManagerAndroid handle allocation/destruction using either Java or the NDK,
+ * whichever is available.
+ */
 class ExternalTextureManagerAndroid {
 public:
 
@@ -47,7 +56,7 @@ public:
     // not quite sure on which thread this is going to be called
     ~ExternalTextureManagerAndroid() noexcept;
 
-    // called on gl thread
+    // called on backend thread
     backend::Platform::ExternalTexture* create() noexcept;
 
     // called on app thread

filament/backend/src/opengl/ExternalStreamManagerAndroid.h

@@ -31,6 +31,11 @@ typedef struct ASurfaceTexture ASurfaceTexture;
 
 namespace filament {
 
+/*
+ * ExternalStreamManagerAndroid::Stream is basically a wrapper for SurfaceTexture.
+ *
+ * This class DOES DEPEND on having a GLES context, because that's how SurfaceTexture works.
+ */
 class ExternalStreamManagerAndroid {
 public:
     using Stream = backend::Platform::Stream;
@@ -40,8 +45,14 @@ public:
 
     Stream* acquire(jobject surfaceTexture) noexcept;
     void release(Stream* stream) noexcept;
+
+    // attach Stream to current GLES context
     void attach(Stream* stream, intptr_t tname) noexcept;
+
+    // detach Stream to current GLES context
     void detach(Stream* stream) noexcept;
+
+    // must be called on GLES context thread, updates the stream content
     void updateTexImage(Stream* stream, int64_t* timestamp) noexcept;
 
 private:

filament/backend/src/opengl/PlatformEGLAndroid.cpp

@@ -20,7 +20,7 @@
 #include "OpenGLContext.h"
 
 #include "android/ExternalTextureManagerAndroid.h"
-#include "android/ExternalStreamManagerAndroid.h"
+#include "ExternalStreamManagerAndroid.h"
 #include "private/backend/VirtualMachineEnv.h"
 
 #include <android/api-level.h>