Add setRelativeMode for SDL driver (#123)

include/ICursorControl.h

@@ -160,6 +160,9 @@ namespace gui
 		\param rect: A pointer to an reference rectangle or 0 to disable the reference rectangle.*/
 		virtual void setReferenceRect(core::rect<s32>* rect=0) = 0;
 
+		//! Internally fixes the mouse position, and reports relative mouse movement compared to the old position 
+		/** Specific to SDL */
+		virtual void setRelativeMode(bool relative) {};
 
 		//! Sets the active cursor icon
 		/** Setting cursor icons is so far only supported on Win32 and Linux */

source/Irrlicht/CIrrDeviceSDL.h

@@ -146,6 +146,12 @@ namespace irr
 			void setPosition(s32 x, s32 y) override
 			{
 				SDL_WarpMouseInWindow(Device->Window, x, y);
+
+				if (SDL_GetRelativeMouseMode()) {
+					// There won't be an event for this warp (details on libsdl-org/SDL/issues/6034)
+					Device->MouseX = x;
+					Device->MouseY = y;
+				}
 			}
 
 			//! Returns the current position of the mouse cursor.
@@ -169,6 +175,17 @@ namespace irr
 			{
 			}
 
+			virtual void setRelativeMode(bool relative) _IRR_OVERRIDE_
+			{
+				// Only change it when necessary, as it flushes mouse motion when enabled
+				if ( relative != SDL_GetRelativeMouseMode()) {
+					if ( relative )
+						SDL_SetRelativeMouseMode( SDL_TRUE );
+					else
+						SDL_SetRelativeMouseMode( SDL_FALSE );
+				}
+			}
+
 		private:
 
 			void updateCursorPos()