a series of CL to add RenderTarget support for external textures (#9504)

* add support for AHardwareBuffer to the java bindings

Texture.setEXternalIamge() now can take a AHArdwreBuffer Java object
as a parameter.

* add an API to set the priority of the Skybox

by default the skybox is always drawn last (priority 7) in order to
reduce overdraw. however, when depth culling is not enabled, it
needs to be drawn first. The new Builder::priority() allows to set
an arbitrary priority for the skybox.

* add rendertarget support for external textures

This was in fact mostly already supported, we just were artificially
preventing that usage. It is supported by the EGL_external_image
extension.

It's the responsibility of the caller/user to not attempt to use an
incompatible format, which has undefined behavior.

FIXES=[466395306]

* add a new android sample to test the AHardwareBuffer as render target

---------

Co-authored-by: Powei Feng <powei@google.com>

android/filament-android/src/main/cpp/SkyBox.cpp

@@ -63,6 +63,14 @@ Java_com_google_android_filament_Skybox_nBuilderColor(JNIEnv *,  jclass,
     builder->color({r, g, b, a});
 }
 
+extern "C"
+JNIEXPORT void JNICALL
+Java_com_google_android_filament_Skybox_nBuilderPriority(JNIEnv *,  jclass,
+        jlong nativeSkyBoxBuilder, jint priority) {
+    Skybox::Builder *builder = (Skybox::Builder *) nativeSkyBoxBuilder;
+    builder->priority(uint8_t(priority));
+}
+
 extern "C" JNIEXPORT jlong JNICALL
 Java_com_google_android_filament_Skybox_nBuilderBuild(JNIEnv *env, jclass type,
         jlong nativeSkyBoxBuilder, jlong nativeEngine) {