Fixed race condition where Android touch events could get scaled by a render target's viewport

src/render/SDL_sysrender.h

@@ -25,6 +25,7 @@
 
 #include "SDL_render.h"
 #include "SDL_events.h"
+#include "SDL_mutex.h"
 #include "SDL_yuv_sw_c.h"
 
 /* The SDL 2D rendering system */
@@ -172,6 +173,7 @@ struct SDL_Renderer
     /* The list of textures */
     SDL_Texture *textures;
     SDL_Texture *target;
+    SDL_mutex *target_mutex;
 
     Uint8 r, g, b, a;                   /**< Color for drawing operations values */
     SDL_BlendMode blendMode;            /**< The drawing blend mode */