Added love.touch module and touch screen events. Resolves issue #825.

Note: this implementation is slightly different from the implementation in the love 0.9.x Android and iOS ports!

Added love.touchpressed(id, x, y), love.touchmoved(id, x, y, dx, dy), and love.touchreleased(id, x, y, dx, dy).

id is a unique identifier for the touch press that persists until love.touchreleased. After that it is no longer unique.
x and y are the coordinates of the touch event in pixels within the window.
dx and dy are the amount in pixels that the touch has moved since the last event.

Added love.touch.getTouchCount and love.touch.getTouch(index).

love.touch.getTouch takes an index (not the same as an id. indices are not stable and don't correspond to a unique touch press.)
It returns the id of the touch and its current x and y coordinates in pixels.

--HG--
branch : minor

src/modules/touch/sdl/Touch.h

@@ -0,0 +1,70 @@
+/**
+ * Copyright (c) 2006-2015 LOVE Development Team
+ *
+ * This software is provided 'as-is', without any express or implied
+ * warranty.  In no event will the authors be held liable for any damages
+ * arising from the use of this software.
+ *
+ * Permission is granted to anyone to use this software for any purpose,
+ * including commercial applications, and to alter it and redistribute it
+ * freely, subject to the following restrictions:
+ *
+ * 1. The origin of this software must not be misrepresented; you must not
+ *    claim that you wrote the original software. If you use this software
+ *    in a product, an acknowledgment in the product documentation would be
+ *    appreciated but is not required.
+ * 2. Altered source versions must be plainly marked as such, and must not be
+ *    misrepresented as being the original software.
+ * 3. This notice may not be removed or altered from any source distribution.
+ **/
+
+#ifndef LOVE_TOUCH_SDL_TOUCH_H
+#define LOVE_TOUCH_SDL_TOUCH_H
+
+// LOVE
+#include "touch/Touch.h"
+
+// C++
+#include <map>
+
+// SDL
+#include <SDL_events.h>
+
+namespace love
+{
+namespace touch
+{
+namespace sdl
+{
+
+class Touch : public love::touch::Touch
+{
+public:
+
+	virtual ~Touch() {}
+
+	virtual int getTouchCount() const;
+	virtual TouchInfo getTouch(int index) const;
+
+	// Implements Module.
+	virtual const char *getName() const;
+
+	// SDL has functions to query the state of touch presses, but unfortunately
+	// they are updated on a different thread in some backends, which causes
+	// issues especially if the user is iterating through the current touches
+	// when they're updated. So we only update our touch press state in
+	// love::event::sdl::Event::convert.
+	void onEvent(Uint32 eventtype, const TouchInfo &info);
+
+private:
+
+	// All current touches, indexed by their IDs.
+	std::map<int64, TouchInfo> touches;
+
+}; // Touch
+
+} // sdl
+} // touch
+} // love
+
+#endif // LOVE_TOUCH_SDL_TOUCH_H