Manually position and scale the content

Position and scale the content "manually" instead of relying on the
renderer "logical size".

This avoids possible rounding differences between the computed window
size and the content size, causing one row or column of black pixels on
the bottom or on the right.

This also avoids HiDPI scale issues, by computing the scaling manually.

This will also enable to draw items at their expected size on the screen
(unscaled).

Fixes #15 <https://github.com/Genymobile/scrcpy/issues/15>

app/src/scrcpy.c

@@ -136,7 +136,7 @@ event_watcher(void *data, SDL_Event *event) {
             && event->window.event == SDL_WINDOWEVENT_RESIZED) {
         // In practice, it seems to always be called from the same thread in
         // that specific case. Anyway, it's just a workaround.
-        screen_render(&screen);
+        screen_render(&screen, true);
     }
     return 0;
 }