Added hints SDL_HINT_MOUSE_DOUBLE_CLICK_TIME and SDL_HINT_MOUSE_DOUBLE_CLICK_RADIUS to allow tuning double-click sensitivity.

Also increased the default double-click radius to 32 pixels to be more forgiving for touch interfaces

include/SDL_hints.h

@@ -262,6 +262,16 @@ extern "C" {
  */
 #define SDL_HINT_GRAB_KEYBOARD              "SDL_GRAB_KEYBOARD"
 
+/**
+ *  \brief  A variable setting the double click time, in milliseconds.
+ */
+#define SDL_HINT_MOUSE_DOUBLE_CLICK_TIME    "SDL_MOUSE_DOUBLE_CLICK_TIME"
+
+/**
+ *  \brief  A variable setting the double click radius, in pixels.
+ */
+#define SDL_HINT_MOUSE_DOUBLE_CLICK_RADIUS    "SDL_MOUSE_DOUBLE_CLICK_RADIUS"
+
 /**
  *  \brief  A variable setting the speed scale for mouse motion, in floating point, when the mouse is not in relative mode
  */