implement some stuff to make exaple SDL app run

NOTE: the main addition in this commit is WIP support for apps which
render on an EGL surface obtained using ANativeWindow_fromSurface

currently, this EGL surface is obtained by creating a 700x700 pixel
window with GLFW (the 700x700 size is hardcoded in several places)
and only Wayland is supported

ideally, we'd want to use a wayland subsurface to position the EGL
surface above the Surface widget it's associated with (and do
whatever for X11)

src/api-impl/android/content/res/AssetManager.java

@@ -339,7 +339,8 @@ public final class AssetManager {
             }
         }
         throw new FileNotFoundException("Asset file: " + fileName);*/
-		return null; // FIXME
+		throw new IOException("FIXME: 'public final AssetFileDescriptor openFd(String fileName)': throwing an exception, which makes e.g SDL2 fall back to using something that we actually have implemented");
+//		return null; // FIXME
     }
 
     /**