osd: show left arrow when playing backwards

If playback direction is backwards, display a triangle
pointing to the left as status symbol of OSD.

player/osd.c

@@ -412,6 +412,8 @@ void get_current_osd_sym(struct MPContext *mpctx, char *buf, size_t buf_size)
             sym = OSD_CLOCK;
         } else if (mpctx->paused || mpctx->step_frames) {
             sym = OSD_PAUSE;
+        } else if (mpctx->play_dir < 0 ) {
+            sym = OSD_REV;
         } else {
             sym = OSD_PLAY;
         }

sub/osd.h

@@ -126,6 +126,7 @@ enum mp_osd_font_codepoints {
     OSD_BRIGHTNESS = 0x0A,
     OSD_HUE = 0x0B,
     OSD_BALANCE = 0x0C,
+    OSD_REV = 0x0D,
     OSD_PANSCAN = 0x50,
 
     OSD_PB_START = 0x10,