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/Canvas.h

@@ -53,18 +53,14 @@ public:
 	void setDepthSampleMode(Optional<CompareMode> mode) override;
 	ptrdiff_t getHandle() const override;
 
-	love::image::ImageData *newImageData(love::image::Image *module, int slice, int x, int y, int w, int h) override;
+	love::image::ImageData *newImageData(love::image::Image *module, int slice, int mipmap, const Rect &rect) override;
+	void generateMipmaps() override;
 
 	int getMSAA() const override
 	{
 		return actualSamples;
 	}
 
-	int getRequestedMSAA() const override
-	{
-		return requestedSamples;
-	}
-
 	ptrdiff_t getRenderTargetHandle() const override
 	{
 		return renderbuffer != 0 ? renderbuffer : texture;
@@ -114,7 +110,6 @@ private:
 
 	GLenum status;
 
-	int requestedSamples;
 	int actualSamples;
 
 	size_t textureMemory;