Fix Issue #278: Set default filter mode for new images.

Add the somewhat lengthy named love.graphics.getDefaultImageFilter() and
love.graphics.getDefaultImageFilter(). The verbose name should point to the
fact that changing the default filter midway through the game will not affect
the images already loaded using love.graphics.newImage().

src/modules/graphics/opengl/Image.h

@@ -78,7 +78,7 @@ namespace opengl
 		*
 		* @param file The file from which to load the image.
 		**/
-		Image(love::image::ImageData * data);
+		Image(love::image::ImageData * data, const Image::Filter& f = Image::Filter());
 
 		/**
 		* Destructor. Deletes the hardware texture and other resources.
@@ -121,7 +121,7 @@ namespace opengl
 		*
 		* @param mode The filter mode.
 		**/
-		void setFilter(Image::Filter f);
+		void setFilter(const Image::Filter& f);
 
 		Image::Filter getFilter() const;