[android] SurfaceView compatible snapshot api

[android] #5587 - snapshot API on surface view, create Bitmap with a ByteBuffer

[android] #5587 - write bitmap to external storage

PreMultipliedImage approach

callback implemented

clean up code

introduce bitmap reuse

cleanup

renaming surfaceContent to bitmap

platform/android/src/native_map_view.cpp

@@ -194,6 +194,35 @@ void NativeMapView::render() {
 
     map->render();
 
+    if(snapshot){
+         snapshot = false;
+
+         // take snapshot
+         const unsigned int w = fbWidth;
+         const unsigned int h = fbHeight;
+         mbgl::PremultipliedImage image { w, h };
+         MBGL_CHECK_ERROR(glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, image.data.get()));
+         const size_t stride = image.stride();
+         auto tmp = std::make_unique<uint8_t[]>(stride);
+         uint8_t *rgba = image.data.get();
+         for (int i = 0, j = h - 1; i < j; i++, j--) {
+            std::memcpy(tmp.get(), rgba + i * stride, stride);
+            std::memcpy(rgba + i * stride, rgba + j * stride, stride);
+            std::memcpy(rgba + j * stride, tmp.get(), stride);
+         }
+
+         // encode and convert to jbytes
+         std::string string = encodePNG(image);
+         jbyteArray arr = env->NewByteArray(string.length());
+         env->SetByteArrayRegion(arr,0,string.length(),(jbyte*)string.c_str());
+
+         // invoke Mapview#OnSnapshotReady
+         env->CallVoidMethod(obj, onSnapshotReadyId, arr);
+         if (env->ExceptionCheck()) {
+             env->ExceptionDescribe();
+         }
+    }
+
     if ((display != EGL_NO_DISPLAY) && (surface != EGL_NO_SURFACE)) {
         if (!eglSwapBuffers(display, surface)) {
             mbgl::Log::Error(mbgl::Event::OpenGL, "eglSwapBuffers() returned error %d",
@@ -436,6 +465,10 @@ void NativeMapView::destroySurface() {
     }
 }
 
+void NativeMapView::scheduleTakeSnapshot() {
+    snapshot = true;
+}
+
 // Speed
 /*
 typedef enum {