android: Make the tiled rendering from the loolwsd actually work.

Not to break the 'old' Android app, introduce a bool that can indicate
if we are using the LOK from the 'old' (LOK-via-JNI-based) or from the
'new' (loolwsd-based) app.

Change-Id: I38bd665cc1d5bc88018574171443ecabc46763df
Reviewed-on: https://gerrit.libreoffice.org/70678Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>

desktop/source/lib/init.cxx

@@ -2349,6 +2349,13 @@ static void doc_setPartMode(LibreOfficeKitDocument* pThis,
     }
 }
 
+#if defined(ANDROID)
+/// For the distinction if the LOK is used for the 'old' (JNI-based) or the
+/// 'new' (loolwsd-based) app.  Default to the 'new', ie. not used from JNI as
+/// implemented in sal/android/libreofficekit-jni.c.
+bool android_lok_from_jni = false;
+#endif
+
 static void doc_paintTile(LibreOfficeKitDocument* pThis,
                           unsigned char* pBuffer,
                           const int nCanvasWidth, const int nCanvasHeight,
@@ -2403,10 +2410,15 @@ static void doc_paintTile(LibreOfficeKitDocument* pThis,
 #else
     ScopedVclPtrInstance< VirtualDevice > pDevice(nullptr, Size(1, 1), DeviceFormat::DEFAULT) ;
 
-#if !defined(ANDROID)
-    // Set background to transparent by default.
-    pDevice->SetBackground(Wallpaper(COL_TRANSPARENT));
+#if defined(ANDROID)
+    if (!android_lok_from_jni)
 #endif
+    {
+        // Set background to transparent by default.
+        // [Unless it is the 'old' (JNI-based) Android app - no idea why it
+        // needs avoiding this.]
+        pDevice->SetBackground(Wallpaper(COL_TRANSPARENT));
+    }
 
     pDevice->SetOutputSizePixelScaleOffsetAndBuffer(
                 Size(nCanvasWidth, nCanvasHeight), Fraction(1.0), Point(),

sal/android/libreofficekit-jni.c

@@ -37,6 +37,7 @@ extern const char* data_dir;
 extern const char* cache_dir;
 extern void* apk_file;
 extern int apk_file_size;
+extern bool android_lok_from_jni; ///< for the distinction if the LOK is used for the 'old' (JNI-based) or the 'new' (loolwsd-based) app
 AAssetManager* native_asset_manager;
 
 extern void Java_org_libreoffice_android_Bootstrap_putenv(JNIEnv* env, jobject clazz, jstring string);
@@ -157,6 +158,11 @@ jboolean Java_org_libreoffice_kit_LibreOfficeKit_initializeNative
     size_t data_dir_len;
 
     (void) clazz;
+
+    // the 'old' app needs to avoid setting the virtual device to transparent
+    // in paintTile(), so indicate we are using the 'old' app
+    android_lok_from_jni = true;
+
     libreofficekit_initialize(env, dataDir, cacheDir, apkFile, assetManager);
 
     // LibreOfficeKit expects a path to the program/ directory