test: opengl/filters: release leaking picture

Fix the following leak:

    Direct leak of 440 byte(s) in 1 object(s) allocated from:
        #0 0x7f16238e1369 in __interceptor_malloc /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_malloc_linux.cpp:69
        #1 0x7f162362bb1b in picture_NewFromFormat ../../src/misc/picture.c:271
        videolan#2 0x55a72c5485c7 in test_opengl_offscreen ../../test/modules/video_output/opengl/filters.c:158
        videolan#3 0x55a72c547005 in main ../../test/modules/video_output/opengl/filters.c:294
        videolan#4 0x7f162314c84f  (/usr/lib/libc.so.6+0x2384f) (BuildId: 2f005a79cd1a8e385972f5a102f16adba414d75e)

test/modules/video_output/opengl/filters.c

@@ -213,6 +213,8 @@ static void test_opengl_offscreen(
 
     vlc_gl_ReleaseCurrent(gl);
     vlc_gl_Delete(gl);
+
+    picture_Release(picture);
 }
 
 int main( int argc, char **argv )