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/OpenGL.h

@@ -328,6 +328,7 @@ public:
 	bool isPixelShaderHighpSupported() const;
 	bool isInstancingSupported() const;
 	bool isDepthCompareSampleSupported() const;
+	bool isSamplerLODBiasSupported() const;
 
 	/**
 	 * Returns the maximum supported width or height of a texture.
@@ -363,6 +364,8 @@ public:
 	 **/
 	float getMaxAnisotropy() const;
 
+	float getMaxLODBias() const;
+
 
 	void updateTextureMemorySize(size_t oldsize, size_t newsize);
 
@@ -407,6 +410,7 @@ private:
 
 	bool pixelShaderHighpSupported;
 	float maxAnisotropy;
+	float maxLODBias;
 	int max2DTextureSize;
 	int max3DTextureSize;
 	int maxCubeTextureSize;