gameactivity: don't discard stylus touchscreen inputs

Modify default_motion_filter to use bitwise AND (&) instead of equality
(==) when checking SOURCE_TOUCHSCREEN. This ensures stylus inputs (which
combine touchscreen and stylus flags) are not discarded, while still
filtering non-touch events.

I noticed touch inputs weren't working on my Galaxy S9 tab device which
includes a stylus.

Changelog-Fixed: Fix touch inputs on tablets with a stylus
Signed-off-by: William Casarin <jb55@jb55.com>

Author: jb55

android-activity/game-activity-csrc/game-activity/native_app_glue/android_native_app_glue.c

@@ -264,7 +264,7 @@ static bool default_key_filter(const GameActivityKeyEvent* event) {
 
 static bool default_motion_filter(const GameActivityMotionEvent* event) {
     // Ignore any non-touch events.
-    return event->source == SOURCE_TOUCHSCREEN;
+    return event->source & SOURCE_TOUCHSCREEN;
 }
 
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