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/Graphics.h

@@ -113,6 +113,7 @@ namespace opengl
 	private:
 
 		Font * currentFont;
+		Image::Filter currentImageFilter;
 		DisplayMode currentMode;
 
 		float lineWidth;
@@ -332,6 +333,11 @@ namespace opengl
 		**/
 		void setColorMode (ColorMode mode);
 
+		/**
+		 * Sets the current image filter.
+		 **/
+		void setDefaultImageFilter(const Image::Filter& f);
+
 		/**
 		* Gets the current blend mode.
 		**/
@@ -342,6 +348,11 @@ namespace opengl
 		**/
 		ColorMode getColorMode();
 
+		/**
+		 * Gets the current image filter.
+		 **/
+		const Image::Filter& getDefaultImageFilter() const;
+
 		/**
 		* Sets the line width.
 		* @param width The new width of the line.