Add mipmapping support to Canvases.

- Added an optional mipmap index argument to the non-table variant of love.graphics.setCanvas, and an optional ‘mipmap’ field to the table variant.
- Canvas:setMipmapFilter now works.
- Added Canvas:generateMipmaps.
- Added Canvas:getMipmapMode.
- Added a new ‘mipmaps’ enum field to the table passed into love.graphics.newCanvas. Accepted values are “none” (default), “manual”, and “auto”.

If a Canvas has the manual mipmap mode, you will either need to render to a mipmap level or call Canvas:generateMipmaps. If it has the auto mode, mipmaps will be automatically generated after rendering to that Canvas. Generating mipmaps is not free.

--HG--
branch : minor

src/modules/graphics/opengl/Image.h

@@ -86,8 +86,6 @@ private:
 
 	size_t textureMemorySize;
 
-	static float maxMipmapSharpness;
-
 }; // Image
 
 } // opengl